318 METAZOAN PHYLA 



duced a larva somewhat like a frog tadpole in form. It possesses a 

 typical notochord which is largely confined to the tail and which extends 

 only as far forward as the pharynx. The pharynx has but a small num- 

 ber of slits. There is a dorsal neural tube, which is dilated at the anterior 

 end into a simple brain. At the anterior end of its body the larva pos- 

 sesses adhesive papillae, a pair of eyes, and otocysts which are comparable 

 to the otocysts, or primitive ear cavities, of vertebrates. When the organ- 

 ism, after a short, active, free-swimming life, is ready to metamorphose, 

 it attaches itself to a solid object by the adhesive papillae (Fig. 219); 

 the tail, notochord, and most of the neural tube disappear, and the brain 

 degenerates into a simple ganglion. The sense organs are lost. While 

 the tail is disappearing, the mouth is gradually changing its location 

 from the point of attachment to the free end of the body, moving up 

 along the dorsal side. This results in a great increase in the extent of 

 the ventral surface, and since there is a corresponding decrease in the 



£ye 

 /Verv^ core/ 



^ofochorc/ 



Stomach „^^„^ . 



nearr per/ branchial 



ves/c/e 



Fig. 219. — Section through the anterior part of the body of a tunicate larva, after 



fixation. From the right side. (From Delage and Herouard, " Traite de Zoologie Concrke," 



after Kowalevsky .) 



dorsal surface, the anal opening and the mouth are brought near to one 

 another, causing the alimentary canal to become U-shaped. The atrial 

 cavity develops from invaginations of the ectoderm on the two sides of 

 the body. These invaginations (peribranchial vesicles) come to open 

 into each other and to surround the whole pharynx ; the external opening 

 of the cavity becomes the atrial funnel. The outer wall of the atrial 

 cavity is the mantle or body wall proper. Finally the cellulose tunic is 

 secreted by the ectoderm of the mantle and the animal has ceased to have 

 any resemblance to the active tadpole-like larva, which bore the promise 

 of a highly developed adult. The number of pharyngeal slits increases 

 and becomes multiplied by the appearance of partitions, so that the wall 

 of the pharynx becomes quite sievelike. 



It is thus clear that in the metamorphosis of the tunicate a very pro- 

 nounced degeneration has taken place; indeed, the fixed adult upon 

 superficial examination seems little higher than a sponge. Some fixed 

 tunicates which reproduce by budding form colonies having a common 

 tunic. These colonies may be spread over the surface of rocks and have 

 somewhat the appearance of a piece of pinkish fat pork. 



