*m PHYLUM CHORDATA $6 



The first beginnings of the atrial cavity appear as a pair of 

 invaginations of the ectoderm which grow inwards and form a 

 pair of pouches, each opening on the exterior by an aperture. 

 There is a difference of opinion as to some points in the history 

 of these atrial pouches, and it remains uncertain to what^extent 

 the ectoderm and endoderm respectively share in the formation 

 of the atrial cavity. Eventually spaces, into the formation of 

 which the two ectodermal diverticula at least largely enter, grow 

 round the pharynx and give rise to the atrial cavity ; and perfora- 

 tions, the stigmata, primarily two in number, place the cavity 

 of the pharynx in communication with the surrounding space. 

 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 

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

 (Fig. 744). In general shape it bears some resemblance to a 



FIG. 744. Free-swimming larva of Ascidia mammillata , lateral view. adh. adhesive papilla ; 

 ali. alimentary canal ; atr. atrial aperture ; cil. gr. ciliated diverticulum, becoming ciliated 

 funnel ; end. endostyle ; eye, eye ; med. nerve-cord (ganglion of trunk) ; noto. notochord ; 

 oto. otocyst ; sens. ves. sense-vesicle ; stig. earliest stigmata. (From Korschelt and Heider, 

 after Kowalevsky.) 



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 

 enclosed in a layer of cells. Parallel with this runs, on the dorsal 

 side, the narrow caudal portion of the nerve-cord, and at the sides 

 are bands of muscular-fibres. In the trunk the nerve-cord is 

 dilated to form the ganglion of the trunk, and, further forwards, 

 expands into the sense- vesicle (sens, ves.) with the otocyst or 

 statocyst (oto.) and eye (eye). A. prolongation of it unites, as already 

 stated, with the ciliated diverticulum from the anterior part of 

 the pharynx. From the walls of this at a later stage are 

 developed, on the dorsal side, the ganglion, and, on the ventral, 

 the neural gland ; the pharyngeal opening (oil. gr.) becomes the 



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