SHELLS. 241 



according to Cuvler, form a group under the name of 

 Acephala nuda, arranged immediately after the 

 testaceous Acephala. Cuvier divides the order into 

 two families, viz. : — 



The first family of the Acephala nuda heing the 

 Segregata, and embracing the genera whose indi- 

 viduals are isolated and without mutual organic 

 connection, although they often live in societies. Only 

 one genus represents this family, viz. : — Genus 

 AscidicB. The Ascidias live in the sea, fixed to rocks, 

 shells, or marine plants. The species are Ascidia 

 rustica and Ascidia lobifera. 



The second family of the Acephala nuda com- 

 prise the Aggregata. These are more or less ana- 

 logous to the AscidicB, but are always united, and 

 constitute a common mass by their union. The 

 Pyrosoma is the only known genus found here 

 belonging to this family. The animals of this 

 genus are gelatinous and transparent, and, placed 

 horizontally in the sea, appear capable of executing 

 slight movements. They are very phosphorescent, 

 and during the darkness of night often exhibit 

 masses of floating light of the most brilliant and 

 varying colours. A small species is known in 

 our harbours {Pyrosoma Atlanticum), in which the 

 animals are arranged in very regular rings. 



