482 



TUNICATA. 



found by VAN BENEDEN and JULIN (No. 7) in Ascidians. The peribranchial 

 sacs arise from paired ectodermal imaginations (cl) which in all respects 



FIG. 250. A, dorsal aspect of a bud of the first asexual type of Anchinia ; B, median 

 section through an older stage (after BARROIS). cl, atrial rudiment (paired in A) ; 

 es,' endostyle ; i, branchial aperture ; m, stomach ; n, nervous system ; oe, oeso- 

 phagus ; p, pericardial rudiment ; ph, pharynx. 



recall the atrial vesicles of the Ascidian larva and the corresponding rudiments 

 in the embryo of Pyrosoma. These paired ectodermal invaginations increase 



in size and come into contact with 

 two lateral, wing-like outgrowths 

 of the pharynx. The close ap- 

 position of the walls of these 

 outgrowths and those of the peri- 

 branchial sacs leads to the forma- 

 tion of the branchial lamellae in 

 which the gill-clefts soon break 

 through. The atrial cavity proper 

 results from a fusion of the two 

 ectodermal invaginations resem- 

 bling that which takes place in 

 the Ascidians (p. 366). A further 

 agreement with the course of 

 development in the Ascidiacea is 

 found in the manner in which the 

 pericardial vesicle develops (Fig. 

 250 B, p), this vesicle being here 

 also a derivative of the entoderm. 

 FIG. 251.-Portion of a colonial tube of U is originally a solid outgrowth 



Dolchinia with its zooids (after KOROT- of the pharynx in the centre of 



NEFF). a, points of attachment of older 



zooids ; c, colonial tube ; g, buds giving 



rise to the sexual individuals ; g', primitive 



bud of the sexual gemmae ; u, wandering 



primitive buds ; z, zooid. 



which a lumen soon forms which 

 becomes connected with the 

 pharyngeal cavity, so that the 

 whole rudiment now represents a 



diverticumm of the pharynx corresponding in position to the posterior end 



