CONCHIFERA. 375 



centre. If tlie external orifice is irritated, the animal to which 

 it belongs alone contracts ; but, if the centre be touched, they 

 all shrink at once. 



In Pyrosoma,* the second family, the animals are aggregated 

 together in great numbers, so as to form a hollow cylinder, open 

 at one end but closed at the opposite, which swims in the sea by 

 the combined contractions and dilatations of all the individuals 

 composing it. The branchial sacs here open upon the exterior 

 of the cylinder, while the anal orifices are in its internal cavity. 

 Thus, a Pyrosoma might be described as consisting of a great 

 number of stars of Botrylli piled one above the other, the whole 

 mass remaining free and capable of locomotion. Many of these 

 moving aggregations of Tunicata emit in the dark a most brilliant 

 phosphorescent light, whence the derivation of the name by which 

 they are distinguished. 



In all other forms of these aggregated Mollusca, which are desig- 

 nated by the general name of Polyclinum^ as in ordinary Ascidi- 

 ans, the anus and branchial orifices are approximated, and placed at 

 the same extremity of the body. They are all fixed; some spreading 

 like fleshy crusts over submarine substances, others forming conical 

 or globular masses, or occasionally so grouped as to produce an 

 expanded disc resembling a flower or an Actinia ; but, whatever 

 the general arrangement of the common mass, it is composed of 

 numerous associated individuals, every one of them corresponding 

 more or less closely as regards their internal structure with the de- 

 scription above given of the organization of Salpse and Ascidians. 



CHAPTER XXII. 



CONCHIFERA (Lamarck) ; ACEPHALES TEST ACES (Cuv.) 



THE great majority of Mollusks which inhabit bivalve shells con- 

 stitute a very numerous and extensive class, distinguished by certain 

 characters possessed by them in common. Encased in dense and 

 massive coverings of such construction as to preclude the possi- 

 bility of their .maintaining more than a very imperfect intercourse 



* ri/, -9s t jire ; ffuftai, a body. -f- vol.vs, many ; xXivn, a bed. 



