SUBPHYLUM A. HEMICHORDATA.

The subphylum includes three genera, Balanoglossus[19], Cephalodiscus and Rhabdopleura; and perhaps a fourth, Phoronis.

The skeletal structures found in Balanoglossus[20] are all endoskeletal. They include:

(1) The notochord. This arises as a diverticulum from the alimentary canal which grows forwards into the proboscis and extends beyond the front end of the central nervous system. It is hypoblastic in origin and arises in the same way as does the notochord of Amphioxus. Its cells become highly vacuolated and take on the typical notochordal structure[21]. The cavity of the primitive diverticulum becomes obliterated in front, but behind it opens throughout life into the alimentary canal.

(2) The axial skeletal rods. These are a pair of chitinous rods which lie ventral to the notochord and in the collar region unite to form a single mass.

(3) The branchial skeleton. The gill bars separating the gill slits from one another are strengthened by chitinous rods in a way closely similar to that in Amphioxus. But between one primary forked rod and the next there are two secondary unforked rods—not one, as in Amphioxus.

(4) The chondroid tissue. This is of mesoblastic origin and may be regarded as an imperfect sheath for the notochord.

In Cephalodiscus and Rhabdopleura as in Balanoglossus the notochord forms a small diverticulum growing forwards from the alimentary canal into the proboscis stalk.

Recent researches on Phoronis[22] show the existence in the collar region of the larva (Actinotrocha) of a paired organ, which is regarded by its discoverer as representing a double notochord.

SUBPHYLUM B. UROCHORDATA (Tunicata).

Skeletal structures of epiblastic and hypoblastic origin occur in the Urochordata. Most Tunicates are invested by a thick gelatinous test which often contains calcareous spicules, and serves as a supporting organ for the soft body. The cells of this test are mesodermal in origin.

In larval Tunicata and in adults of the group Larvacea the tail is supported by a typical notochord, which is confined to the tail. In all Tunicata except Larvacea all trace of the notochord is lost in the adult.

SUBPHYLUM C. CEPHALOCHORDATA.

This subphylum includes the well-known genus Amphioxus[23]. In Amphioxus the skeleton is very simple. It contains no trace of cartilage or bone and remains throughout life in a condition corresponding to a very early stage in Vertebrata. The skeleton of Amphioxus is partly hypoblastic, partly mesoblastic in origin.

(a) Hypoblastic skeleton.

The notochord (fig. 3, 2) is an elastic rod extending along the whole length of the body past the anterior end of the nerve cord. It lies ventral to the nerve cord, and shows no trace of segmentation. It is chiefly made up of greatly vacuolated cells containing lymph, but near the dorsal and ventral surfaces the cells are less vacuolated. The notochord is immediately surrounded by a structureless cuticular layer, the chordal sheath, and outside this comes the mesoblastic skeletogenous layer, which also surrounds the nerve cord.

The branchial skeleton. This consists of a series of chitinous elastic rods which strengthen the gill bars and are alternately forked and unforked ventrally. The forked rods are primary, and are U-shaped in section, the unforked rods are secondary, and are circular in section. All these rods are united at intervals by transverse rods.

(b) Mesoblastic skeleton.

The buccal skeleton. On each side of the mouth there is a curved bar resembling the notochord in structure. The bars are segmented, and each segment bears a smaller rod which supports a tentacle, the whole forming the buccal skeleton (fig. 3, 4).

The notochord is enclosed in a thick sheath of connective tissue continuous with a thinner sheath round the nerve cord. The sheaths of the notochord and nerve cord together form the skeletogenous layer, and prolongations of it form the myomeres or septa between the myotomes or segments of the great lateral muscles of the body.

The skeleton of each median fin consists of small cubical masses of a gelatinous substance arranged in rows (fig. 3, 1 and 7), and serving to strengthen the fins.