526 THE ANATOMY OF INVERTEBRATED ANIMALS. 



a third component, derived from the indifferent tissue, out of 

 which the reproductive organs of the parent have been de- 

 veloped. 



In Amouroiicium proUferum^ agamic multiplication takes 

 place when the larva has fixed itself and grown into a soli- 

 tary ascidian. The long post-abdomen (as the prolongation 

 of the abdomen beyond the alimentary canal is termed) sepa- 

 rates itself from the body, carrying with it the heart, and 

 divides into a number of segments which rise to the summit 

 of the test of the parent, range themselves around it, and be- 

 come converted into independent zooids. The parent devel- 

 ops a new heart and post-abdomen. The process appears to 

 be repeated in the post-abdomina of the new zooids. The 

 post-abdomen is a process of the ectoderm, the inner cavity 

 of which is divided by a septum into two chambers, contain- 

 ing many fatty cells. The septum itself incloses a cavity, 

 and there appears to be no doubt that it is a prolongation cf 

 the pharyngeal sac. When the segments of the post-abdo- 

 men develop, the cavity of the anterior end of the septum 

 dilates and divides, as in Didemnum^ into three chambers, of 

 which the median becomes the branchial sac, and the lateral 

 the atrial chambers. The rest remains as the septum of the 

 post-abdomen of the foetus, and its cavity at first communi- 

 cates with the branchial sac, between the endostyle and the 

 oesophageal aperture. 



Kovvalewskj' ^ has observed the formation of buds from 

 free cellular masses in the common test of Didemnxiin styli- 

 ferimi ; the origin of these masses is undetermined. They 

 multiply by division, after the rudiments of the alimentary 

 cavity and of the reproductive organs have made their ap- 

 pearance. The alimentary cavity gives off a process whence 

 the oesophagus, stomach, and intestine are developed, and 

 then becomes divided by longitudinal partitions into three 

 chambers, a median and two lateral. The latter give rise to 

 the lateral chambers of the atrium, which subsequently open 

 into one another on the neural side of the body, and finally 

 communicate with the exterior by a median atrial opening. 



Gegenbaur'^ has described the detachment of the ova of 

 a species oi Didemnum into the substance of the common test, 

 where they are developed into caudate larvae provided with 

 an eye. Before the development of the larva is nearly com- 



'"Ueber die KKOspuiiff der Ascidien." (" Archiv fur Mikr. Anatomie," 

 1874.) 



\ " Uebcr Didemnum gelatinosumy (" Arcliiv fur Anat.," 1862.) 



