490 TUNICATA. 



Salpce, are endowed with a power of locomotion ; but subsequently they 

 become aggregated in groups, either incrusting foreign bodies, or else, 

 uniting together to form a mass of definite shape, they seem to enjoy, to 

 a certain extent, a community of action. They are arranged by Cuvier* 

 in three principal groups, distinguished by the following characters. In 

 the first (BotryUus^}, the little bodies of the individual animals are 

 ovoid ; but they fix themselves upon the exterior~of sea- weed or other 

 substances in regular bunches, consisting of ten or twelve, arranged 

 like the rays of a star around a common centre. The branchial orifices 

 in such are all placed around the circumference of the star, while the 

 excretory apertures open into a common cavity in the centre. If the 

 external orifice is irritated, the animal to which it belongs alone con- 

 tracts ; but if the centre be touched, they all shrink at once. 



(1286.) 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 fee 

 described as consisting of a great number of stars of Botrylli piled one 

 above the other, the whole mass remaining free and capable of locomo- 

 tion. 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. 



(1287.) In all other forms of these aggregated Mollusca, which are 

 designated by the general name of Polydinum, as in ordinary Asci- 

 dians, 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 

 disk resembling a flower or an Actinia ; but, whatever the general 

 arrangement of the common mass, it is composed of numerous asso- 

 ciated individuals, every one of them corresponding more or less closely, 

 as regards their internal structure, with the description above given of 

 the organization of Salpae and Ascidians. 



* Begne Animal, vol. iii. p. 168. t /3orpus, a bunch of grapes. 



| trvp (-os), fire ; <nJ/ta, a body. TroXus, many ; K\ivrj, a bed. 



