368 TUNICATA. 



that the ova are formed by the mantle itself in the localities where 

 they are generally met with. Future investigations, conducted 

 under more favourable circumstances, will no doubt reveal the 

 existence of some internal ovarian nidus, in which the eggs are 

 first developed, and from whence they are subsequently removed 

 to the branchial membranes ; as we shall find hereafter to be the 

 usual arrangement in other forms of bivalve Mollusca. 



CHAPTER XXI. 



TUNICATA.* 



THE singular class of Mollusca to which the name at the head 

 of this chapter has been applied, is at once distinguished by the 

 remarkable character afforded in the texture of the external invest- 

 ment of the body. In their general organization the Tunicata 

 are very nearly allied to the ordinary inhabitants of bivalve shells, 

 with which, both in the structure and arrangement of their viscera, 

 they correspond in many particulars ; but, instead of being enclosed 

 in any calcareous covering, a strong flexible cartilaginous or coria- 

 ceous integument forms a kind of bag encasing their entire body, 

 and only presenting two comparatively narrow orifices, through 

 which a communication with the exterior is maintained. 



Various are the forms under which these animals present them- 

 selves to the eye of the naturalist ; but the enumeration of them 

 will be more conveniently entered upon hereafter. We shall, 

 therefore, at once lay before the reader the principal points con- 

 nected with the structure and habits of an Ascidia belonging to 

 one of the most perfectly organized families ; and, after examining 

 this attentively, our descriptions of allied genera will be rendered 

 more simple and intelligible. The Ascidians are abundantly met 

 with upon the shores of the ocean, especially at certain seasons of 

 the year. In their natural condition they are found fixed to the 

 surfaces of rocks, sea- weed, or other submarine bodies ; frequently, 

 indeed, they are glued together in bunches, but in this case in- 

 dividuals are simply agglomerated without organic union. In- 

 capable of locomotion, and deprived of any external organs of sense, 

 few animals seem more helpless or apathetic than these apparently 



* Tunicatus, clad in a tunic. 



