OF THE MOLLUSC01DES, OR TUNICATA. 



523 



zoophytes. They all have a distinct digestive tube, turned on 

 itself, and open at both extremities, and have a very well 

 developed branchial apparatus (Fig. 551) ; most of them also 

 present vestiges of a nervous system, but have no ganglionary 

 ring like the mollusca, properly so called ; finally, almost all 

 multiply by granul ttions as well as by ova, and thus form 

 aggregations of* individuals more or less completely confounded 

 with each other. 



These animals are all aquatic, and are formed on two prin- 

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

 aria or ciliated polypi. 



Fig. 550. Plumatella.* 



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

 a very large mantle, in the form of a sac (Fig. 551), which 

 constitutes in front of the abdomen or visceral mass a respi- 

 ratory cavity, enclosing branchia?, variously arranged. They 

 have a heart, and bloodvessels, in which the nourishing liquid 

 circulates in a very singular manner, for the current changes 



* a, group of the plumatellae of the natural size ; b, others magnified, and 

 seen in different positions ; c, termination of the intestine. 



