TUNICATA. 479 



unless brought in contact with the male fluid containing spermatozoa, 

 whereas in gemmiparous reproduction such a concurrence is by no 

 means necessary ; neither germinal vesicle nor any male apparatus is 

 required. 



CHAPTER XIX. 

 TUNICATA*. 



(1259.) 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 by the texture of the external investment 

 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 coriaceous 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. 



(1260.) Yarious are the forms under which these animals present 

 themselves 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 connected with the 

 structure and habits of an Ascidia belonging to one of the most per- 

 fectly organized families ; and after examining this attentively, our 

 descriptions of allied genera will be rendered more simple and intelli- 

 gible. The Ascidians are abundantly met with upon the shores of the 

 ocean, especially at certain seasons of the year. In their natural con- 

 dition 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 individuals are simply agglomerated, without 

 organic union. Incapable of locomotion, and deprived of any external 

 organs of sense, few animals seem more helpless or apathetic than these 

 apparently shapeless beings, and the anatomist is surprised to find 

 how remarkably the beauty and delicacy of their interior contrasts with 

 their rude external appearance. In the species selected for special 

 description (Phallusia nigrd), the external envelope (fig. 243, a a a) is 

 soft and gelatinous in its texture, fixed at its base to a piece of coral (I), 

 and exhibiting at its opposite extremity two orifies (&, /), placed upon 

 prominent portions of the body. Through the most elevated of these 

 orifices (h) the water required for respiration and the materials used 

 * Tunicatus, clad in a tunic. 



