Eighth Section. 



Tunicata, 



General Review. 



§ 299. 



In conceding to the division of the Tunicata, formerly by general 

 consent reckoned among the Mollusca, the rank of a special phylum 

 of the Animal Kingdom, we do no more than assign its legitimate value 

 to the very peculiar organisation of these animals. Their pecu- 

 liarities separate them not only from all the classes of the Mollusca, 

 but also from all the other animal phyla, although it must be ad- 

 mitted that they present certain distant affinities to some Vermes 

 (the Enteropneusti), and that equally close relationships are readily 

 to be observed between them and the lowest Vertebrata. We shall 

 return to the subject of these relationships in treating of the Verte- 

 brata; hero it is only necessary to remark that the absence of a 

 clearly-marked metamerism of the body forbids the intimate asso- 

 ciation of the two groups, although indications of the formation of 

 metameres in certain regions of the body may be pointed out in 

 some Tunicata. 



In the position of the most important organs and their primitive 

 relations we find the most obvious indications of Vertebrate affinity. 

 Thus we have the enteron, with its foremost division adapted to the 

 functions of a respiratory organ. A movable appendage of the body, 

 present in the adult state in only one division, but in others in 

 the larval stage, contains an organ of support, which exhibits the 

 closest resemblance to the primitive axial skeleton of the Vertebrata. 

 A further general characteristic of the group is seen in the hyaline 

 body-covering, which often attains a considerable development, and 

 is known as the " mantle." 



