210 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



A 



the so-called cyathozooid, corresponds to the solitary form of Doliolum 

 and of Salpa (fig. 7). In Pyrosoma the solitary generation is reduced 

 to a transient, embryonic stage. This cyathozooid gives rise very 

 early to four buds, or primary ascidiozooids, which in turn bud and 

 form a great number ol secondary ascidiozooids; and thus the 

 colony is established. 



The stolon of the cyathozooid develops very early as a prominent, 

 sac-like outgrowth from that end of the embryo which lies opposite to 



the cloacal aperture (figs. 

 6 and 7 ) . This outgrowth 

 soon becomes marked off 

 into four short segments, 

 by partial constrictions. 

 Each of these segments is 

 destined to develop into 

 a primary ascidiozooid. 

 The stolon, originally 

 straight, becomes curved 

 to the left, so that by fur- 

 ther growth it comes to 

 encircle equatorially the 

 yolk of the cyathozooid. 

 The buds all lie with their 

 ventral surfaces distal 

 from the parent. 



The earliest organs of 

 the chain of buds origi- 

 nate for the most part as 

 direct continuations of the 

 corresponding organ rudi- 

 ments in the cyathozooid. 

 The ectoderm is directly 

 continuous in the two. 

 The enteric canal is a tu- 

 bular outgrowth running 

 throughout the chain. 

 There is found in the cy- 

 athozooid the rudiment of an endostyle-fold on the outer layer of 

 endoderm, and this also is prolonged into the stolon. In addition 

 to these, are a pair of peribranchial tubes, a transient pericardial 

 tube on the right side of the body, and two strands of mesoblast 

 cells. The nervous system has a new origin in each bud, forming as 

 an invagination of the dorsal ectoderm. 



During the growth of its buds the ectoderm of the cyathozooid 

 secretes a layer of cellulose around itself and its buds, just within the 



A 



,-777. 



Fig. 7.— Diagrams to snow tue position of tue organs 

 in a Pyrosoma cyathozooid (A) and a Salpa (£), 



SOLITARY FORM. AFTER KoRSCHELT AND HEIDER 

 (1900). 



