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.
[a]Fig. 3. Diagram of the skeleton of] Amphioxus lanceolatus × 3
(after a drawing in the Index collection at the Brit. Mus.).
| 1. skeleton of dorsal fin. | 5. branchial skeleton. |
| 2. notochord. | 6. septa separating the |
| 3. neural tube. | myotomes. |
| 4. buccal skeleton. | 7. skeleton of ventral fin. |
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.