430 MOLLUSC A. 



wards, at first live and swim separately, only becoming united 

 at a certain subsequent period, a fact which is in direct oppo- 

 sition to this opinion. 



Their branchiae, as in the Ascidise, form a large sac, tra- 

 versed by the aliment before it arrives at the mouth ; their 

 principal ganglion is also situated between the mouth and the 

 arms; a nearly similar disposition obtains with respect to the 

 viscera and ovary(l). 



Notwithstanding this, some of them, like the Biphora, have 

 an opening at each extremity. Such is the 



BoTRYLLus, Gsert., 



Of an oval form, fixed on various bodies, and united by tens or 

 twelves, like the fays of a star. The branchial orifices are at the 

 external extremities of these rays, and the anus terminates in a com- 

 mon cavity, which is in the centre of the star. If an orifice l)e irri- 

 tated, but a single animal contracts; if the centre be touched they all 

 contract. These very small animals attach themselves to certain 

 Ascidiae, Fuci, Sec. (2) 



In some particular species, three or four stars appeared to be 

 piled one on the other(3). 



Pyrosoma, Peron. 



The Pyrosomae unite in great numbers, forming a large hollow 

 cylinder, open at one end and closed at the other, which swims in 

 the ocean by the alternate contraction and dilatation of the indivi- 

 dual animals which compose it. The latter terminate in a point on 

 the exterior, so that the whole external surface of the tube is bris- 

 tled with them,; the branchial orifices are pierced near these points, 

 and the anus debouches in the internal cavity of the cylinder. A 

 Pyrosoma may thus be compared to a great number of stars of Bo- 

 trylli strung together, the whole of which is movable(4). 



(1) It is to M. de Savigny that we are indebted for our recent knowledg-e of the 

 singular organization of the whole of this family, formerly confounded with the 

 Zoophytes properly so called. At the same time, Messrs Desmarets and Lesueur 

 made known the particular structure of the BotryUi and Pyrosomae. See tiie ex- 

 cellent work of M. Savigny in his Mem. surles anim. sans verteb., part II, fasc. I. 



(2) See Desmai'ets and Lesueur, Bullet, des Sc. May lSl5;—BotryUus stellaius, 

 Gaertner, or Alcyonium Schlosseri, Gm., Pall., Spic. Zool., X, iv, 1 — 5. 



(3) Botryllus conglomeratus, G3ert., or Alcyonium conglovieratum, Gm.; Pall., 

 Spic. Zool. X, iv, 6. 



(4) See Desmarets and Lesueur, loc. cit. 



