3'4 



MANUAL OF ZOOLOGY 



nervous system as in Balanoglossus. They both differ from Balano- 

 glossus in having the alimentary canal bent on itself, so that the anal 

 opening is situated not far from the mouth, in the presence of tentacles 

 arising from the collar; and in the comparatively small size of the 

 proboscis. 



2. THE UROCHORDA 



Still more unlike a vertebrate in general appearance than Balano- 

 glossus, and yet, as the earlier stages show, indubitably to be assigned 

 to the Chordate phylum, are the Asoidians or Sea-squirts and their 

 allies. 



Sea-squirts are familiar objects on rocky sea- 

 shores, where they occur often in large associa- 

 tions, adhering firmly to the surface of the rock. 

 They also live free in sand and in mud, at differ- 

 ent depths, some being deep-sea forms. When 

 touched the Ascidian ejects with considerable 

 force two fine jets of sea-water, which are found 

 to proceed from two apertures on its upper end. 

 The shape of the Ascidian, however, can only 

 be profitably studied in the case of specimens 

 that are completely immersed in the sea-water, 

 specimens not so immersed always undergoing 

 contraction. In an uncontracted specimen l 

 (Fig. 197) the general shape is that of a short 

 cylinder with a broad base by which it is fixed 

 to the rock. The free end presents a large 

 rounded aperture, and some little distance from 

 it on one side is a second of similar character. 

 The former aperture is termed the oral, the 

 latter the atrial. A strong current of water 

 will be noticed, by watching the movements of 

 floating particles, to be flowing steadily in at the former and out of the 

 latter. When the animal is removed from the water both apertures 

 become narrowed, so as to be almost completely closed, by the contrac- 



FlG. 197. — Ascidia, en- 

 tire animal seen from 

 the right - band side. 

 (After Herdman.) 



1 The European species, whose anatomy is here described, is approxi- 

 mately represented by our common large Ascidia callosa, which lives in 

 deep water off the Maine coast. 



