XIII 



PHYLUM CHORDATA 



37 



cyst (oto.) and eye (eye). A })iulong;iti()n of it unites, as ali-eady 

 stated, with the ciliated diverticiUum 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 {ril. gr.) becomes the 

 ciliated funnel. The enteric caual is distinguishable into pharynx, 

 oesophagus, stomach, and intestine. The pharynx opens on the 

 exterior by the mouth : in its ventral floor the endostyle {end.) 

 has become developed ; its walls are pierced by stigmata, the 

 number of which varies. The atrial cavity has grown round the 

 pharynx, and opens on the exterior by a single aperture only 

 {atr.). The heart and pericardial cavity have been formed. In 

 this tailed free-swimming stage the larva remains only a few 

 hours ; it soon becomes fixed by the adhesive papilla) and begins 



m-e-cL atr 



sens.x/e^ ciLar- 



dh. 



clU 



sti^ 



e^& 



Fk;. 735.— Free-swimming larva of Ascidia mammillata, lateral view. a<lh. adhesive papilli« ; 

 all. alimentary canal ; atr. atrial ai^erture ; i-'A. 'jr. ciliated diverticulum, Vjecoming ciliated 

 funnel ; end. endostvle ; ei/e, eye ; med. nerve-cord (ganglion of trunk) ; noto. notochord ; oto. 

 otocyst ; sem. ves. sense-vesicle ; stiri. earliest stigmata. (From Korschelt and Ileider, after 

 Kowalevsky.) 



to undergo the retrogressive metamorphosis by which it attains the 

 adult condition. 



The chief changes involved in the retrogressive metamorphosis 

 (Fig. 736) are the increase in the number of pharyngeal stigmata, 

 the diminution, and eventually the complete disappearance, of the 

 tail with the contained n&tochord and caudal part of the nerve- 

 cord, the disappearance of the eye and the otocyst, the dwindling 

 of the central part of the nervous system to a single ganglion, and 

 the formation of the reproductive organs. Thus, from an active 

 free-swimming larva, with complex organs of special sense, 

 and provided with a notochord and well-developed nervous 

 system, there is a retrogression to the fixed inert adult, in 

 which all the parts indicative of affinities with the Yertebrata 

 have become aborted. The significance of these facts will be 

 considered when we come to discuss the general relationships of 

 the Chordata. 



In some simple Ascidians, and in the composite forms in which 

 development takes place within the body of the parent, the meta- 

 morphosis may be considerably abbreviated, but there is always, 



