ASCIDIAE SALPAEFORMES. 



43 



continuous with the rudiment of the genital organs of the parent, 

 gives rise to the peribranchial (atrial) cavity, to the central nervous 

 system (n), to the gonads, and to the mesodermal structures of the 

 budding zooid. The pharyngeal diverticulum becomes the pharynx and 

 develops the intestine as an outgrowth (Fig. 37). It is clearly homo- 

 logous with the epicardium found in many synascidians. The pericardium 

 is developed as a vesicle formed of mesodermal tissue on the right side. 

 It comes to lie on the dorsal side of the pharyngeal process which projects 

 into the stolon of the fully formed zooid (Fig. 37 pc), and thus differs in 



et d 



FIG. 37. A chain of three zooids in the budding of Pyrosoma (from Korschelt and Heider 

 after Seeliger), I youngest, II middle and III oldest bud (nearly developed). D point at 

 which the pharyngeal process of the parent enters ; d pharyngeal process (epicardium) ; 

 dc rudiment of digestive canal ; dm elongated cell mass (? phosphorescent and haemato- 

 poietic organs) ; e rudiment of atrial aperture ; eb elaeoblast ; EC ectoderm of parent ; oc ecto- 

 derm of new stolon ; Es endostyle of parent ; es endostyle of bud ; fg ciliated pit ; g ganglion ; 

 h testis ; fid intestine ; hz heart ; i rudiment of mouth ; Id atrium ; ks gill-clefts ; If internal 

 longitudinal gill bars : lm phosphorescent organ ; m stomach ; ms genital strand ; n rudi- 

 ment of nervous system ; o ovary (egg follicle with egg) ; oe oesophagus ; p dotted line in- 

 dicating the boundary of the atrial cavity (peribranchial sac) ; pc pericardium ; rz languete ; 

 t tentacle rudiment ; v tube connecting the enteron of the first and second buds. 



position from the pericardium of other forms, which is placed ventral to 

 the epicardium (see p. 15). The nervous system develops from the distal 

 end of the genital cord (Fig. 36) which passes round the front end of the 

 pharyngeal process on to the dorsal surface of the bud and becomes hol- 

 lowed out to form a vesicle. The ganglion arises from the thickened dorsal 

 wall of this vesicle, the cavity of which acquires a tubular communication 

 with the pharynx and forms the subneural gland and duct (Fig. 37, fg) 

 The elaeoblast also develops from the mesoderm, as paired masses which 



