OF THE MOLLUSC01DES, OE TUNICATA. 



479 



cipal types the tunicata, properly so called, and the bryozo- 

 aria or ciliated polypi. 



614. The tunicata, properly so called, are provided with 

 a very large mantle, in the form of a sac (Fig. 465), which con- 

 stitutes in front of the abdomen or visceral mass a respiratory 

 cavity, enclosing branchiae, variously arranged. They have 

 a heart, and bloodvessels, in which the nourishing liquid cir- 

 culates in a very singular man'ner, for the current changes 



Fig. 4&i. Plumatella.* 



its direction periodically, so that in the space of some minutes 

 the same canal performs the function of an artery and a vein. 

 In this class are arranged the biphores (Fig. 465), the pyro- 

 soma, and the ascidiae (Fig. 157), distinguished into simple 

 and aggregated. These last have often a phytoid appearance. 

 The history of the biphores presents a very remarkable 

 peculiarity. Successive generations do not resemble each 

 other, but are composed alternately of aggregated and solitary 

 individuals. The first are hermaphrodite, and produce each 

 a young one, which lives isolated, but which has no repro- 

 ductive organs, and gives birth, by granulation, to a sort of 



* a, group of the plumatellse of the natural size ; b, others, 

 seen in different positions ; c, termiuati n of the intestine. 



ignified, and 



