262 ELEMENTARY ZOOLOGY 



provided with two openings, one for the ingress and the 

 other for the exit of water, a strong current of which flows 

 constantly through the body. By means of this current 

 the ascidian obtains food. Usually sea-squirts live 

 together in large colonies, and in some cases a number of 

 individuals enclose themselves in a common gelatinous 

 mass, forming what is called a compound ascidian. 



The ascidian when born is a tiny, free-swimming, tad- 

 pole-like creature with a slender finned tail. It swims 

 about freely for only a few hours, however, soon attach- 

 ing itself to a rock, and in its further development becom- 

 ing degenerate. It loses its tail and with it the short 

 notochord possessed by the larva; the eye and the auditory 

 organ are lost, and the nervous system and alimentary 

 canal become much reduced and simplified. Sea-squirts 

 in their adult stage are very simple degenerate animals, 

 with low functional development, yet their embryonic and 

 larval conditions show a considerable degree of structural 

 specialization, and the presence of the notochord in these 

 early stages reveals their affinity with the backboned 

 animals. 



