ACEPHALA NUDA. 285 



FAMILY II. 



AGGREGATA. 



The second family consists of animals more or less analogous to 

 the Ascidias, but united in a common mass^ so that they seem to 

 communicate organically with each other, and in this respect to 

 connect the Mollusca with the Zoophytes; but independently of their 

 peculiar organization, these animals, according to the observations 

 of Messrs Audouin and Milne Edwards, at first live and swim sepa- 

 rately, only becoming united at a certain subsequent period, a fact 

 which is in direct opposition to this opinion. 





BOTRYLLUS, 



An oval form, fixed on various bodies, and united by tens or twelves, like 

 the rays of a star. The branchial orifices are at the external extremities of 

 these rays, and the opening terminates in a common cavity, which is in the 

 centre of the star. If an orifice be irritated, but a single animal contracts; 

 if the centre be touched they all contract. These very small animals at- 

 tach themselves to certain Ascidise, Fuci, &c. 



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

 on the other. 



PYROSOMA, Peron. 



The Pyrosomse 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 individual animals which compose 

 it. The latter terminate in a point on the exterior, so that the whole ex- 

 ternal surface of the tube is bristled with them; the branchial orifices are 

 pierced near these points. A Pyrosoma may thus be compared to a great 

 number of stars of Botrylli strung together, the whole of which is movable. 



The Mediterranean and the Ocean produce large species, the animals of 

 which are arranged with but little regularity. They exhibit a phosphores- 

 cent appearance during the night. 



A smaller species is also known where the animals are arranged in very 

 regular rings. 



The species of the remainder that are known are all fixed, and till 

 now have been confounded with the Alcyonia. The visceral bundle 

 of each individual is more or less extended into the common cartila- 

 ginous or gelatinous mass, more or less narrowed or dilated in cer- 



"'*. 



