MONOSCIOUS MOLLUSCA. 



341 



Fig. 69. 



%>'*>- ."" 



aid of its tail, like the tadpole." Moving about in this 

 manner for a few hours or days, the time probably regulated 

 by the temperature of the circumfluent water, or by speci- 

 fical ordination, the time comes when 

 the law of nature compels it to settle 

 and unfold the character of its species. 

 The head is then applied to the site 

 chosen for the future evolution ; it 

 enlarges and adheres by three short 

 budding radicles ; and, curiously enough, 

 the tail which, during the process of 

 fixation, had been held upright and 

 stationary, is now moved violently, as 

 if the creature were struggling to re- 

 gain its liberty. " At this juncture the 

 vibrations of the tail become so rapid, 

 that, like those of a cord in tension, 

 its figure is hardly discernible by the 

 eye. At length quiescence follows ; 

 some diffusing matter escapes from the 

 margin of the flattened head, and the 

 spinula is rooted irreversibly to the 

 spot." The growth goes on — a dark 

 nucleus is substituted for the adherent 

 head, the base enlarges irregularly, the 

 tail is absorbed and disappears, and 

 two nipples, the beginnings of the oral 

 and anal apertures, rise from the sur- 

 face, and complete the metamorphosis 

 of the tadpole moving larva into the mammiform fixed 

 Ascidia.* Thus, you observe, that, by giving them active 

 and unconfined larvae, the Creator diffuses the species ; and 

 to these fixed animals gives possession of a tract of coast 

 not less extensive than that occupied by their nomacle 

 relations. 



Savigny, whose opinions are always entitled to respectful 

 consideration, believed that the egg of the compound Tuni- 

 cata was itself a composite body, organized and developing 

 itself in the pattern peculiar to each species. He appears to 

 have been led to this remarkable conclusion by the exami- 

 nation more especially of the reproductive germs of the 

 Pyrosoma. In this animal the eggs, when still very minute, 

 detach themselves from the ovary in succession, one by one, 

 and place themselves between the intestine and the bottom 



* Rare and Remarkable Anim. Scot. ii. io<>, 15] 



