By the term sea-squirts, the naturalist denominates some of the most remarkable animals which it is his province to study. In more polite phraseology the sea-squirts are termed ‘Ascidians,’ this appellation being derived from the Greek askos, meaning a wine-skin or Eastern leather-bottle, to which, in outward shape and form the sea-squirts bear a very close resemblance. And as a final designation, the animals under discussion may be known as ‘Tunicates,’ since their bodies are inclosed within a tough bag or ‘tunic,’ the chemical composition of which forms, as we shall presently shew, one of the notable points of their structure. The sea-squirts present themselves to the zoologist as a group of beings exhibiting many exceptions to the ordinary rules of animal organisation; and it may also be noted that they have attained a degree of scientific fame almost exceeding that which their most ardent admirers could have expected. The young sea-squirt has thus been credited in certain scientific speculations with presenting the naturalist and mankind at large with a fac-simile of the early progenitor and far-back ancestor of the whole vertebrate group of animals, including man himself—in other words, it is maintained that the young sea-squirt, through some peculiar process of modification and elevation, has given origin to the highest group of living beings. With the promise before us of obtaining information regarding a most interesting group of animals, which are thus held by some savants to possess relations of no ordinary kind to man himself, the reader will require little incentive to follow out the steps of a brief inquiry into their life-history and relations.

The fame of the sea-squirts is by no means of modern date. Aristotle gives us a succinct description of them in his History of Animals under the designation ‘Tethea;’ and by the same name Pliny has made the sea-squirts of classic reputation, since we learn from this latter author that they were included as articles of importance in the pharmacopoeia of the Romans. In their commonest phases, the sea-squirts appear as little leathery bags of clear aspect, through the somewhat transparent wall of which the internal organs can be discerned. The resemblance of the animals to the ancient wine-skin has already been remarked. The wine-skin, as every one knows, was made of the stomach of some animal, or of the skin so arranged as to present two orifices or necks. Thus when we look at a common sea-squirt we see a veritable little ‘leather-bottel,’ measuring from half an inch to an inch or more in length, attached by one extremity to the rock at low-water mark, or to the shell we have dredged, and bearing on its upper surface two prominent openings, each supported on a short neck. The origin of the common name of ‘sea-squirt’ is by no means hard to trace. The incautious observer who picks up a sea-squirt which has through unpropitious fate been cast up on the sea-beach after a storm, after a short survey of the sac-like body, may possibly be tempted to squeeze it as a preliminary to further investigation. On being thus irritated, the animal will most likely retaliate by forcibly ejecting jets of water from the two orifices of the ‘bottel;’ this procedure possibly resulting in the relinquishment of the sea-squirt as altogether an unlikely and unfavourable object for further study. But the observation of this unpolite habit on the part of the animal, will be found to assist our further comprehension of its physiology, and of the manner in which the functions of its life are carried on.

A highly curious item of sea-squirt history is furnished at the outset by the consideration of the rough bag or ‘test’ in which its organs are inclosed. When the chemist analyses this part of the animal, he finds it to be composed in greater part of a substance known as cellulose. It so happens that cellulose is a most important constituent of plants, being almost as common in vegetables as starch. Hence zoologists accounted it a strange and unwonted proceeding on the part of an animal, that it should manufacture in a seemingly natural manner a substance proper and peculiar to the plant-world. The multiplication of cases of like kind in animals has destroyed the novelty and unique nature of the sea-squirt’s case; but none the less curious must the fact be accounted, that the animal should mimic the plant in the mode and results of its life. When the tough outer sac is cut open, we come upon a much more delicate and softer structure, known as the mantle. This latter forms an inner lining to the test, and is the structure upon the presence of which the sea-squirt’s power of ejecting water depends. The mantle is a highly muscular layer, and lies next the organs and internal belongings of the animal.

The clearest mode of describing the structure of the sea-squirt is that of beginning with that neck of the bottle-shaped body on which the mouth opens. The mouth leads, curiously enough, not into a throat, but into a large chamber, named the breathing-sac. The walls of this chamber may be simply described as composed of a network of fine blood-vessels; the meshes of this network being provided with those delicate vibratile filaments, named cilia, the function of which is to keep up, by their movements, a constant circulation of the water admitted to the breathing-chamber. Just within the mouth-opening a few small tentacles or feelers exist, these organs serving to guard the entrance to the body. On the floor of the breathing-sac an opening may be perceived; this aperture leading into the throat, and being, therefore, by many naturalists termed the true mouth. And in the way of digestive apparatus, we find the sea-squirt to possess a stomach, intestine, and other organs.

It is highly interesting to note the manner in which the sea-squirt obtains its food. The nutritive wherewithal consists of sedimentary matters, such as minute animals and plants, these substances being drawn into the breathing-sac along with the currents of water which are continually being taken into the body. The nutritive sediment is collected together by certain folds of the lining membrane of the breathing-chamber, and is thus transferred to the mouth-opening below. The breathing-chambers of the sea-squirts, it may be noted in passing, frequently afford lodgment to tiny dwellers in the shape of little pea-crabs. The writer has noticed these little lodgers to issue forth at night from the mouth of the sea-squirts, when the latter have been kept in an aquarium, in order to pick up particles of food on the floor of their abode. The crabs retreated to their shelter on the slightest alarm; and this case of companionship presents one of those curious instances of animal association which at present we are wholly unable to explain.

The food being converted into blood in the digestive system, we may next inquire as to the means which the sea-squirt possesses for circulating the blood through the body. In higher animals, the heart and blood-vessels perform this important task; and in the sea-squirt we find these structures to be represented; the sea-squirt’s heart indeed, in respect of its peculiarity of action, being without a parallel in the whole animal world. The heart consists of a simple tube, from each end of which blood-vessels pass, some being distributed to the breathing-chamber, and others to the body generally. In the highest animals the heart has the double function of driving pure blood through the body, and of circulating impure blood through the breathing organs for purification. It is noteworthy to observe, that by a curious and, as already remarked, altogether unparalleled contrivance, Nature has succeeded in causing the simple tube-heart of the sea-squirt to perform the work done by the complex organ of higher animals. When we observe the movements of the sea-squirt’s heart, we may see it to propel the blood by its pulsations at first to the breathing-chamber for purification. Then a pause succeeds, and the heart is observed to pulsate in the reverse direction, and to drive the blood from the breathing-chamber through the body. Probably no better illustration of the manner in which, by a modification of function, Nature compensates for simplicity of structure, could be had, than that afforded by the sea-squirt’s heart.

The breathing-chamber, as we have seen, receives fresh sea-water from the outside world, this water containing the vivifying oxygen required for the purification and renewal of the blood. Having given up its oxygen to the blood contained in the fine blood-vessels of the breathing-chamber, and its sediment to serve for food, the great bulk of water contained in the breathing-sac has now to be got rid of, to make room for a fresh supply. This process is effected in the most admirable manner through the currents created by the little filaments or cilia, which cause a constant flow of water to pass through the network walls of the breathing-chamber into a second sac or bag which lies parallel with it. This latter sac receives the name of the atrium, and communicates with the outer world by the second neck or orifice of the body. Hence the water which enters by the mouth-opening, after passing through the breathing-chamber, is ejected by the second aperture of the body, and affords the material wherewith the sea-squirt vents its indignation on prying humanity in the shape of the jets d’eau which have procured for it its popular designation. The sea-squirt regarded in relation to its sedentary habits, would seem to require no great exercise of nervous powers. Accordingly we find its nervous system to be represented by a single mass of nervous matter, placed near the mouth, and from which nerves pass to the other parts of the body. The acts of a sea-squirt may thus be regarded as purely of the character we are accustomed to name ‘automatic.’ It is provided with instincts enabling it to carry on the acts of its life and to exhibit a certain degree of irritability, without at the same time knowing the ‘reason why’ of its own actions.

The sea-squirts present no exceptions to the universal rule of Harvey, omne animal ex ovo—this philosopher believing in the universal development of the animal-form from an ovum or egg. But unlike most higher animals, the young sea-squirt does not come from the egg in the likeness of the parent. It first appears as a tadpole-like body, this creature—the larva as it is named—being produced in some thirty hours after the development of the egg begins. The head of the tadpole is provided with pigment spots or rudimentary eyes, and with three little suckers, by means of which it ultimately attaches itself to fixed bodies, prior to assuming the form of the adult and perfect Ascidian. The tail of the tadpole-larva next becomes retracted within its body, and therein disappears, whilst after fixing itself, the well-known features of the sea-squirt become duly developed. A Russian zoologist has remarked that in the tail of the sea-squirt a long rod-like body is to be seen. Now in the lowest fishes, the spine exists in a similar and rod-like condition; and hence, by a certain school of naturalists, it is urged that the vertebrates may have originated in the past from some form resembling the sea-squirt larva, in whose tail we are therefore invited to behold the first stage of the vertebrate backbone or spine. It is noteworthy to observe, however, that the opinions of these naturalists are by no means accepted by the scientific world at large; and one eminent German observer declared that the rod-like body in the sea-squirt larva’s tail was not situated in the back, but in the opposite region of the body, and that therefore it could not be regarded as corresponding to the ‘back’-bone of higher animals.

Certain near relations of the sea-squirt become of exceeding interest from their departure from the more usual and staid type of Ascidian structure. Amongst these errant members of the sea-squirt tribe the most remarkable perhaps are the SalpÆ—clear, gelatinous animals, met with swimming in long connected chains on the surface of the sea in tropical regions. The celebrated novelist Chamisso, author of the charming story, Peter Schlemil or the Shadowless Man, who to his literary tastes united a striking aptitude for natural history research, discovered that the young of these chain-salpÆ invariably appears as a single form; whilst each single salpa has the power of producing a connected chain. Thus the salpa sea-squirts exist in two distinct forms—chain-salpas and single salpas, and to use Chamisso’s own words: ‘A salpa mother is not like its daughter or its own mother, but resembles its grand-daughter and its grand-mother.’

Another curious group of the sea-squirts is that known by the name of the PyrosomÆ, a name literally meaning ‘fiery-bodies.’ These latter forms exist as connected masses of sea-squirts aggregated together, so as to form a hollow cylinder or tube, closed at one end; this animal-colony swimming on the surface of the sea, by the admission and forcible ejection of water from the interior of the tube. Such a means of locomotion reminds us of a veritable hydraulic engine, and is decidedly a useful modification of the common sea-squirt’s habit. The pyrosomas exhibit a strange phosphorescent light, seen also in such animals as the jelly-fishes. These luminous sea-squirts when seen in shoals, have well been described as ‘miniature pillars of fire, gleaming out of the dark sea, with an ever-waning, ever-brightening, soft, bluish light, as far as the eye could reach on every side.’ Side by side with this description from the pen of a distinguished naturalist, may be placed the poetic realisation of a similar scene by Sir Walter Scott, who in the Lord of the Isles has happily noted the luminosity of the sea when,