BRITISH TUNIC ATA. 



are of a brownish colour and very numerous ; so that it some- 

 times happens that in this animal large portions of the blood- 

 system can be traced in a single individual. Most of the 

 information on this portion of the anatomy has been obtained 

 from these three species ; but nevertheless several important 

 points have been verified in the living animal. 



The blood-system in the simple Tunicates may be looked 

 upon as closed, however limited the true vascular portion 

 of it may be. The blood-channels throughout the organism 





FKI. 4. Blood, lacunae (curved) and muscular bands (straight) between the 

 mantle and lining- membrane of Ascidia venosa. Highly magnified. 

 . Blood globules, much more highly magnified, of a reddish-yellow 

 colour and containing a few granules. 



are well denned ; but whether or not they are provided with 

 proper walls, and, if so, to what extent, is not easy to determine. 

 The trunk channels leading to and from the heart have cer- 

 tainly all the appearance of being true vessels ; and the 

 branchial network lias likewise the character of being truly 

 vascular. The blood-channels in the test have also distinct 

 walls ; but in this case they are apparently composed of a 

 prolongation of the mantle or inner tunic. Traces, however, 

 of an inner vessel may be observed in the main trunks ; but 

 this apparent vessel may be nothing more than a continuation 



