CHAPTER XX. 



OF THE CLASS OF TUNICATA. 



968. ALTHOUGH the Mollusca are in general possessed of a 

 calcareous shell, sometimes enveloping the whole body, and 

 sometimes inclosing but a small portion of it, no appearance of 

 such a structure is presented among the animals of the lowest 

 group, the Class TUNICATA. Feeble as are the powers of 

 sensation and locomotion in a great proportion of the Molluscous 

 tribes, they would seem almost extinct among the members of 

 this group. The greater number of them pass their whole lives 

 in one situation, agglutinated by their external tunic to submarine 

 rocks, or attached by a footstalk prolonged from it ; many species 

 associate together, like the Polypifera, to form a compound 

 structure, in which several individuals are united more or less 

 closely ; and those which have no fixed attachment enjoy little 

 independent locomotive power, but are driven about at the 

 mercy of the waves. No beings possessed of a complex internal 

 structure, a distinct stomach and alimentary tube, a pulsating 

 heart and ramifying vascular apparatus, with branchial append- 

 ages for aerating the blood, and highly-developed secretory and 

 reproductive organs, can be imagined to spend the period of 

 their existence in a mode more completely vegetative than these. 



969. The animals of this Class are entirely enveloped in a 

 firm elastic tunic (whence their name), which is always provided 

 with two orifices. The general form of the body, as well as the 

 colour and consistence of this tunic, vary considerably in the 

 different species. Sometimes it is globular or egg-shaped ; some- 

 times narrow and prolonged. The tunic is often of leathery or 

 even cartilaginous firmness, and of a dark colour ; whilst we 

 occasionally find it soft, membranous, and transparent, and of a 

 light greenish tint, so that the clusters of animals look like 



