SALPA DEMOCRATICA-MUCRONATA. 531 



ous forms, but these eventually break up, and the constituent 

 zooids are set free. Fig. 152 shows the two zooids of the 

 species /Salpa democratica-?nucronata, viz., the sexless zooid, 

 Salpa democratica (Fig. 152, I.), and the free sexual zooid, 

 Salpa mucronata (Fig. 152, IT.). 



The recent investigations of Dr. Todaro, 1 in accordance 

 with those of Kowalewskv. show that the stolon is formed, 

 as in Pyrosoma, by the conjunction of a process of the endo- 

 derm which forms the extremity of the endostyle, with an 

 outgrowth of the ectoderm, and with certain cells of the meso- 

 blast. But, according to Todaro, there is this essential differ- 

 ence : the young Salpa?, which make their appearance in 

 double series along the stolon, are developed altogether from 

 the mesoblastic cells. These cells, in fact, besome aggre- 

 gated into masses, of which four are arranged in the circum- 

 ference of each segment into which the stolon is divided ; and 

 two of these masses, one on each side of each segment, are 

 converted into young Salpa3 by a process analogous to that 

 by which a morula becomes an embryo. If this account of 

 the matter be correct, the agamic development of the Salpce 

 would rather resemble that of the germ masses of the sporo- 

 cysts of Trematoda, or the pseud-ova of insects, than ordinary 

 budding. 



Each sexual zooid possesses a testis and a single ovum. 

 The latter is contained in an ovarian follicle, the slender duct 

 of which is attached to the wall of the atrium and opens into 

 the atrial cavity. The testis attains its full growth and func- 

 tional perfection only after the ovum has undergone develop- 

 ment. It follows, therefore, that impregnation must be ef- 

 fected by the spermatozoa of some other zooid. The sexless 

 form which is developed from the egg goes through the early 

 stages of its development in the atrial cavity of the parent, to 

 the walls of which it is attached by a peduncle (Fig. 152, 

 III.), the centre of which is occupied by a diverticulum of the 

 vascular canals of the parent, inclosed within a cup-shaped 

 cavity in free communication with the blood-sinuses of the 

 foetus. It is, in fact, a true placenta ; and, during life, the 

 independence of the fcetal and maternal circulations is readily 

 observed, as the blood-corpuscles of the two organisms course 

 through their respective channels. 



The early stages of the development of the embryo Salpa 

 have been investigated by numerous observers, most recently 



1 " Sopra lo Sviluppo e l'Anatomia delle Salpe," 1S75. 



