472 



The Tunicates, or Ascidians 



like the tadpole of the frog. It has a large oval body and a 

 long tail which lashes about, forcing the animal for^vard with 

 a wriggling motion. Nor is the resemblance superficial; it 

 pervades every part of the structure, as may be seen from the 

 adjacent diagram. The mouth is nearly terminal and com- 

 municates with a gill-chamber provided with gill-clefts. At 

 the posterior end of the gill-chamber begins the alimentary 

 tract, which pursues a convoluted course to the vent. In the 

 tail, but not extending to any distance into the body, is an 

 axial cylinder, the notochord, which here, as in all other verte- 

 brates, arises from the hypoblast ; and above it is the spinal 

 cord (epiblastic in origin), which extends forward to the brain, 

 above the gill-chamber. Besides, the animal is provided with 

 organs of sight and hearing, which, however, are of peculiar 

 construction and can hardly be homologized with the correspond- 

 ing organs in vertebrates. So far the correspondence between 

 the two types is very close, and if we knew nothing about the 



later stages, one would without 

 doubt predict that the adult tuni- 

 cate would reach a high point in 

 the scale of vertebrates. These 

 high expectations are never ful- 

 filled ; the animal, on the contrarv, 

 pursues a retrograde course, re- 

 sulting in an adult whose relation- 

 ship to the true vertebrates never 

 would have been suspected had 

 its embryology remained unknown. 

 After the stage described this 

 retrograde movement begins . From 

 various parts of the body lobes 

 grow out, armed on their extremi- 

 ties with sucking-disks. These 



Fig. 279.-Anatomy of Tunicate. ''^'''^ ^°™*^ ^" contact with some 

 (After Herdman, per Parker A- subaquatic obiect and adhere to 

 Haswell.) .^ „i 



it. ihen the notochord breaks 



down, the spinal cord is absorbed, the tail follows suit, the 



intestine twists around, and the cloaca is formed, the result being 



much like the diagram near the head of this section. In forms 



