320 



MANUAL OF ZOOLOGY 



and pericardial cavity have become developed. In this tailed, free- 

 swimming stage the larva remains only a few hours; it soon becomes 

 fixed by the adhesive papilla, and begins to undergo the retrogressive 

 metamorphosis by which it attains the adult condition. 



The chief changes involved in the retrogressive metamorphosis (Figs. 

 200 and 201) are increase in the number of pharyngeal stigmata, the 

 diminution, and eventually the complete disappearance, of the tail with 

 the contained notochord and caudal part of the nerve-cord, the dis- 

 appearance of the eye and the otocyst, the dwindling of the trunk part 

 of the nervous system to a single ganglion and the formation of the 

 reproductive organs. Thus, from an active, free-swimming larva, with 

 well-developed organs of special sense, and provided with a notochord 

 and well-developed nervous system, there is a retrogression to the fixed 



77».ec? air sens.r/e* cil.aT' 



ali 



slip 



Fig. 200 — Free-swimming larva of Ascidia mammlllata, lateral view, adh, adhe- 

 sive papillae: ali, alimentary canal; atr, atrial aperture: cil. gr, ciliated groove; 

 end, endostyle; eye, eye; med, nerve-cord; ttoto, notochord ; oto, otocyst; sens. 

 ves, sense vesicle ; stig, earliest stigmata. (From Korschelt and Heider, after 

 Kowalewsky.) 



inert adult, in which all the parts indicative of affinities with the Ver- 

 tebrata have become aborted. 



A remarkable feature of the Ascidians is that, though many remain 

 simple, others give rise to colonies by a process of budding. In some 

 of these compound forms, distinguished as the Composite Ascidians, the 

 tests of the zooids are united together to form a mass of gelatinous 

 consistency in which the zooids of the colony lie embedded (Fig. 202). 

 These compound forms, such as Amaroucium, are common on the New 

 England coast in shallow water. 



A minute animal which swims about in the surface waters of the sea 

 has in most respects an extremely close resemblance to the tailed larva 



