OF BIVALVE SHELLS. 459 



delicate fringes which encircle these orifices are often named 

 the cilia or ciliary processes, but more properly the tenta- 

 cular Jilaments. 



Cellulose — a constituent principle of every vegetable — is 

 also found to constitute a material proportion of the mantle 

 of the simple and social Tunicata ; it forms the soft mass in 

 the cavities of which the groups of individuals of the com- 

 pound Tvuiicata are lodged ; and it forms the envelope within 

 which are contained the muscles, the viscera, and the nerves 

 of the Salpae. This vegetable principle is not to be found 

 in any other class or order of animals. The only apparent 

 exception is afforded by the Doliolum mediterraneum of 

 Otto ; but the fact of the existence of cellulose in it may 

 rather prove the animal to be more nearly related to the 

 Salpae than to the Beroides, with which it has been arranged.* 



The Tunicata are all naked or shelless. Mr. Garner has, 

 however, found " calcareous pieces " in some of the simple 

 species. " They consist of two small conical jij^. §3 

 tubes, curiously reticulate in their structure, 

 situated one in each orifice of the cartilagin- -^k 



ous tunic, and projecting from it externally. ^ ^ ^^ 

 The water, &c., must pass through them."f ^, ^^ 



— In some of the compound Tunicata the ^ W ^. 

 common mass is more or less loaded with ^ ^, 

 aggregated crystallisations (Fig. 83) of car- 

 bonate of lime, — perhaps the first index of ^ 



the shell. 



II. TERMINOLOGY OF THE BIVALVES=CONCII^. j; 



A bivalve shell is one composed of two oppositely corre- 

 sponding pieces or valves joined together by a hinge occupy- 

 ing a limited portion of their periphery, and on which they 

 open and shut. 



The shell of the Pholas has been placed amongst multi- 

 valves, because it has a few additional pieces placed over 

 and above the hinge, but it is truly bivalve, these acces- 

 sory pieces having no character of proper valves. The only 

 shells which can perplex you are those which Lamarck has 

 placed in a family denominated Tubicolae, from the circum- 



* " De la Composition ct tie la Structure des Envelopes dcs Tuniciers ; 

 par MM. Lowig et A. Kolliker," in Ann. des Sc. Nat. (1846) v. 193. 



t Cliarleswonh's Mag. Nat. Hist. ii. 579. 



X In what relates to the remainder of this letter, I have borrowed so 

 much from Deshayes' Traite Ele'mentaire, Introd. p. 303 — 367, that I 

 must pay tlie debt by this general acknowledgment. 



