43O THE EVOLUTION OF MAX. 



tiquity, and must have existed during the primaeval 



The whole class to which the Ascidians belong bears the 

 name of Mantle-animals (Tunicata), because the body is 

 enclosed in a thick and firm membrane, as in a mantle or 

 tunic. This tunic, which is sometimes soft and jelly-like, 

 sometimes tough and leather-like, sometimes firm and 

 cartilaginous, is distinguished by many remarkable charac- 

 teristics. Probably the most remarkable of these is, that it 

 consists of a woody mass or cellulose, the same plant-cell 

 material which forms the firm exterior of the cells of 

 plants, the substance of the wood. The Mantle -animals are 

 the only class of animals which really possess a cellulose 

 covering, a wood-like envelope. Sometimes the cellulose 

 tunic is variegated, at other times it is colourless. Not 

 uncommonly it is set with spines or hairs, like a cactus. 

 Many foreign substances, such as stones, sand, fragments of 

 mussel-shells, and so forth, are often embedded in the tunic. 

 The Sea-squirt has, therefore, received the name " micro- 



cosm. 



'117 



In order correctly to understand the internal organiza- 

 tion of the Sea-squirt, and thoroughly to compare it with the 

 Amphioxus, we must place ourselves in the same position to 

 it as to the latter (Plate XI. Fig. 14, on the left side ; the 

 mouth extremity is turned upward, the back to the right, 

 the abdomen to the left). The posterior end, corresponding 

 to the tail of the Amphioxus, is usually adherent, often by 

 means of root-like processes. The ventral and dorsal sides 

 are internally very different, but are often externally undis- 

 tinguishable. On opening the thick tunic, in order to note 

 the internal organization, we observe first a very consider- 



