xiii PHYLUM CHORDATA 31 



The first beginnings of the atrial cavity appear about the same 

 time as a pair of invaginations of the ectoderm which grow 

 inwards and form a pair of pouches, each opening on the exterior 

 by an opening. There is some difference of opinion as to some 

 points in the history of these atrial pouches. According to one 

 account each gives off a diverticulum inwards towards the pharynx, 

 while from the latter a pair of diverticula grow outwards to meet 

 them ; the two sets of diverticula subsequently meet and unite to 

 form a pair of passages, one leading from each atrial pouch to the 

 pharynx ; these form the first pair of stigmata. The atrial pouches 

 then extend round the pharynx until they form a narrow space 

 completely surrounding it, the cavities of the two pouches 

 coalescing, and a number of perforations of the pharynx placing 

 its cavity in direct communication with the surrounding space. 

 According to another account two endodermal diverticula from 

 the primitive pharynx grow out and open into the atrial pouches ; 

 these diverticula subsequently become greatly expanded and grow 

 round the pharynx to form the peri-pharyngeal space. It will be 

 observed that, while according to the former of these two views 

 the peri-pharyngeal space is ectodermal in origin, according to 

 the latter it is endodermal. The two openings of the atrial 

 pouches subsequently coalesce to form one the permanent atrial 

 aperture. 



It will be useful now, at the cost of a little repetition, to sum- 

 marise the various characteristics of the larval Ascidian at the 



FIG. 690. Free-swimming larva of Ascidia mammillata, lateral view. adh. adhesive papillfe ; 

 ali. alimentary canal ; atr. atrial aperture ; cil. gr. ciliated groove ; end. endostyle ; eye, eye ; 

 med. nerve cord ; noto. notochord ; oto. otocyst ; sens. yes. sense-vesicle ; stig. earliest stigmata. 

 (From Korschelt and Heider, after Kowalewsky.) 



stage when it escapes from the egg and becomes free-swimming 

 (Fig. 690). In general shape it bears some resemblance to a 

 minute tadpole, consisting of an oval trunk and a long, laterally- 

 compressed tail. The tail is fringed with a caudal fin, which is 

 merely a delicate outgrowth of the thin test covering the whole 

 of the surface ; running through the delicate fringe are a series of 

 striae, presenting somewhat the appearance of the fin-rays of a 

 Fish's fin. In the axis of the tail is the notochord (noto.\ which 

 at this stage consists of a cylindrical cord of gelatinous substance 



