298 CHARACTERS OF VERTEBRATE ANIMALS 



is interesting on account of its chemical composition, as it is 

 largely made up of a substance, cellulose, which is almost 

 entirely confined to plants. Dissection (fig. 172) fails to show 

 the presence of a notochord, and the central nervous system 

 merely consists of an elongated thickening or ganglion, placed 

 about half-way between the two apertures, but the pharynx is 

 perforated for breathing purposes as in ordinary Vertebrates. 

 The mouth, in fact, leads into a large branchial sac or pharynx, 

 perforated by innumerable small holes and suspended in an 

 atrial cavity which opens by the atriopore already mentioned. 

 There is therefore a general resemblance to the Lancelet in this 

 respect, but in this case the intestine also opens into the atrial 

 cavity. 



Were we to rely only upon the anatomy of the adult we 

 should hesitate before placing the Sea- Squirt among the Verte- 

 brates, seeing that it conforms to only one of the three chief 

 tests; but the matter is set definitely at rest by a study of the 

 development. The &^^ of Ascidia becomes a tadpole - shaped 

 larva in which a notochord is present, though it is confined to 

 the tail, and for that reason is often called a urochord (Gk. 

 oura, tail; chorde, string), which gives the name Urochorda, 

 adopted at the head of this section as the name for the group 

 of Sea- Squirts generally. And further, the ascidian tadpole 

 is possessed of a hollow brain and spinal cord situated on the 

 dorsal side of the body, besides which it may be noted that 

 the perforations in the pharynx are at first of simple character, 

 consisting of paired openings suggestive of the gill -clefts of 

 fishes, &c. 



After leading a free life for some time the tadpole attaches 

 itself by means of adhesive projections situated at the head end, 

 the tail with its urochord gets smaller and smaller and ultimately 

 disappears, while the central nervous system is simplified into 

 a single solid ganglion. We have, therefore, the remarkable 

 phenomenon of an animal which, when young, possesses the 

 distinctive vertebrate characters, but loses most of them in the 

 adult condition, becoming, so to speak, of lower grade. This 

 is a good example of biological degeneration. 



It is a singularly interesting fact that one or two small free- 

 swimming Ascidians, of which the best known {^Appendicularid) 

 (fig. 173) occurs in British seas, retain throughout life the tadpole 



