CHAPTER II. 



UROCHORDA 1 . 



IN the Solitaria, except Cynthia, the eggs are generally laid, and 

 impregnation is effected sometimes before and sometimes after the 

 eggs have left the atrial cavity. In Cynthia and most Caduci- 

 chordata development takes place within the body of the parent, 

 and in the Salpidae a vascular connection is established between the 

 parent and the single foetus, forming a structure physiologically com- 

 parable with the Mammalian placenta. 



Solitaria. The development of the Solitary Ascidians has been 

 more fully studied than that of the other groups, and appears moreover 

 to be the least modified. It has been to a great extent elucidated 

 by the splendid researches of Kowalevsky (Nos. 18 and 20), whose 

 statements have been in the main followed in the account below. 

 Their truth seems to me to be established, in spite of the scepticism 

 they have met with in some quarters, by the closeness of their cor- 

 respondence with the developmental phenomena in Amphioxus. 



The type most fully investigated by Kowalevsky is Ascidia 

 (Phallusia) mammillata; and the following description must be 

 taken as more especially applying to this type. 



The segmentation is complete and regular. A small segmentation 

 cavity appears fairly early, and is surrounded, according to Kowalevsky, 

 by a single layer of cells, though on this point Kupffer (No. 27) and 

 Giard (No. 1 1 ) are at variance with him. 



I The following classification of the Urocliorda is adopted in the present chapter. 

 I. Caducichordata. 



. Solitaria ex. Ascidia. 



A. blMPLICIA o T IT 



Socialia ex. Clavellina. 



B. COMPOSITA Sedentaria BotryUns 



(Natantia ex. Pyrosoma. 



C COVSERTA Sal P idje - 



JDoliolidaj. 



II Perennichordata. 



Ex. Appendicularia. 



