4i6 



THE CIRRIPEDS, TUNICATES AND ECHINODERMS. 



notochord arising from the ectoderm and projecting forward from the anterior 

 lip of the telocoel; the same lateral bands of mesoderm; the same kind of trioculate 

 median eye; the same infolding of the medullary plate on the neural side of the 

 head to form the primitive stomodagum, or subneural gland, and a similar organ 

 the opposite side of the head representing an acl/tal or a potential haemostoma; 



on 



and a similar larval metamorphosis. 



When the tunicate larva escapes from the egg, three glandular tubercles 

 appear on the anterior haemal surface of the head, representing the remnants 

 of arthropod cephalic appendages. After a short, free swimming existence, it 

 attaches itself by these appendages to some foreign object, head down, and in a 

 nearly vertical position, and then begins its metamorphosis. (Fig. 284, A.) It 

 undergoes a partial rotation, turning neural side up; the cephalic tubercles are 

 gradually merged in the larger cephalic stalk; the body contracts, taking on a pro- 



FlG. 284. — Diagrams illustrating the mode of fixation and the metamorphosis of an ascidian. 



nounced curvature that draws the head end upward toward the root of the tail; 

 the latter atrophies, and with the growth of the mantle, the remnants of the body 

 are completely enclosed in the large atrial chamber. 



In Figs. 285 and 286 I have attempted to show, in a purely diagrammatic way, 

 the manner in which a cirriped-like animal could be metamorphosed into a 

 tunicate. These figures should be compared with what actually takes place in a 

 tunicate (Fig. 284), and with the conditions that actually occur in the metamorpho- 

 sis of a typical cirriped, such as Lepas (Fig. 274J, or with that which prevails in 

 the adult condition of more degenerate cirripeds, such as Ibla and Alcippe, 

 Scalpellum, Petrarca, and Sacculina. (Figs. 278 and 280.) 



With the atrophy of the body, the notochord disappears and the elongated 

 nerve cord is reduced to a small, compact cerebral ganglion which, like that in 

 many parasitic cirripeds, still surrounds the proximal end of the old stomodaeum, 

 i.e., the subneural gland, s.g. 



In the cirripeds (Lepas), the anterior end of the enteron forms an immense 

 chamber with numerous enteric pouches arranged in transverse bands. (Fig. 



