ANATOMY AND DEVELOPMENT. 21$ 



elsewhere, is to abbreviate the development by omitting 

 certain ontogenetic processes, and so arriving at the de- 

 sired end, as it were, by a short cut. 



One of the most impressive instances of such an abbre- 

 viated development, and one which can be demonstrated 

 with the utmost certainty, is afforded by the genus Clave- 

 lina, in contrasting it with the closely allied genus dona. 



Clavelina (see Fig. 96) is an Ascidian, provided at its 

 base with creeping processes or stolons containing a lumen 

 continuous with the body-cavity, by which it adheres to 

 rocks and weeds. Buds are formed from the stolon, which 

 grow up into new individuals precisely like the parent form 

 which developed from the egg, and so a colony is produced. 



Ciona also has similar basal processes of the test, con- 

 taining prolongations of the original body-cavity, but no 

 buds are produced. 



In Clavelina, the embryonic development, up to the time 

 of the hatching of the larva, takes place inside the peri- 

 branchial chamber of the parent, which becomes converted 

 into a kind of brood-pouch. 



In Ciona, the eggs are extruded into the water, where 

 they are fertilised by the simultaneous extrusion of sper- 

 matozoa from the same individual. Finally, in Clavelina 

 the egg is much larger and contains more food-yolk than 

 that of Ciona. 



We see, therefore, that in these two genera the egg is at 

 the outset subjected to different sets of conditions, both 

 internally and externally. 



METAMORPHOSIS OF CIONA INTESTINALIS. 



Three stages in the metamorphosis of the larva of Ciona 

 intestinalis are shown in Fig. 105. First, there is the free- 

 swimming larva, which, after a pelagic existence of one or 



