194 TH E ASCIDIANS. 



and its recurrent action, the Tunicate heart would appear 

 to be a unique organ peculiar to the group of the Ascid- 

 ians and analogous but not homologous, or only incom- 

 pletely so, with the heart of the Vertebrates. 



Again, the vascular system of an Ascidian is only func- 

 tionally comparable to that of Amphioxus, since true vessels 

 provided with an endothelial lining are entirely absent, 

 their place being taken by sinuses which arose by reduction 

 from the original body-cavity. 



Renal Organs. 



The renal organs of the Ascidians have no apparent 

 morphological relation to those of Amphioxus, and therefore 

 need not detain us. They consist of a group of bladder-like 

 vesicles with cellular walls lying around the intestine. The 

 products of excretion (uric acid, etc.) are deposited inside 

 the vesicles in the form of solid concretions. There is no 

 excretory duct. In Molgula, there is a single large cylin- 

 drical renal sac closed at both ends and lying on the right 

 side of the body, behind the heart, known as the organ of 

 Bojanus. 



Comparison between an Ascidian and Amphioxus. 



Having sketched in rough outline the organisation of an 

 adult Ascidian, we are now in a position to consider in 

 what respects it resembles and in what it differs from that 

 of Amphioxus. We shall see that some of the most funda- 

 mental differences will be made good by the structure of 

 the larva, such as the absence of a dorsal nerve-tube and 

 of a notochord. 



Let us first consider the resemblances between an adult 

 Ascidian and Amphioxus. 



