CHORDATA 



267 



common chamber with one external opening, as in Pyrosoma 

 ( Fig. 278). 



The fertilized egg of the Tunicata generally develops into a 

 larva called the tadpole larva because, although very small, 

 it resembles in shape the tadpoles, which are the larval stage 

 in the development of the frogs and toads. This tadpole larva 

 is very interesting because it possesses in its tail region a well- 

 developed notochord. In one group which retains in its adult 

 condition the general form of the larva, this notochord persists ; 

 but in all other Tunicata it disappears in the course of develop- 



bds 



end 



or.ap 



FlG. 279. Sa/pa democratica, asexual form, ventral aspect, atr.ap, atrial aperture; branch, 

 dorsal lamina; end, endostyle ; ht, heart; mus.bds, muscle bands; ne gn, nerve ganglion ; 

 proc, processes at the posterior end; sens.org, sensory organ; stol, sloion. (Alter Vogt 

 and Jung, from Parker and Haswell's Text-book.) 



ment and so is absent in the adult. Some of the free-swimming 

 genera have a rather complicated development since there is 

 an alternation of generations. The fertilized ovum develops 

 into a non-sexual individual which possesses a process on the 

 body wall called the stolon ; from this stolon sexual individuals 

 may be produced by budding (Fig. 279). This alternation of 

 generations is very conspicuous in such genera as Salpa and 

 Doliolum. 



SUBTYPE III. CEPHALOCHORDA 



The subtype Cephalochorda (Gr. /ce<£a\»/, head, and x°P^ r J> 

 cord), variously known as Leptocardii (Gr. \e7rT0'?, slender, and 

 Kciphla, heart) and Acrania (Gr. a, without, and Lat. cranium, 

 skull), is represented by a single genus, Amphioxus, the lancelet, 

 which by some zoologists is classified under the next subtype. 



