92 BOTRYLLUS PYROSOMA. 



30. We give the name of BOTRYLLUS to small aggregated 

 Tunicata of an oval form, which differ little from the preceding, 

 except their branchial sack is open at both extremities, and the 

 anal orifice terminates in a central cavity, around which ten or 

 twelve of these mollusks are grouped like the rays of a star. 



Fig. 115. PYROSOMA. 



31. The PYROSOMA 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 animals that compose it. In their individual organization they 

 are similar to the preceding. 



OF THE PARTS OF SHELLS. 



We have now brought our history of the Mollusca to a close ; 

 but with the view of imparting clearer notions on the subject of 

 Conchology, or History of Shells, we shall add here a few 

 words in relation to the parts of shells. It is absolutely neces- 

 sary to understand these parts, before we can comprehend the 

 descriptions given by conchologists, or be able to describe shells 

 in such a way as conchologists can recognise them. 



The most simple farm of a shell is the cone (fg- 70, page 

 57). The apex of the cone is oblique and excentric. In the 

 Limpets (Patella), Argonaut, and Nautilus, the apex is directed 

 towards the head, but in most other mollusks, towards the oppo- 

 site extremity of the body. 



A shell may consist of one piece, as in the inopercular 

 univalves that is, univalves without an operculum or door. 



A shell may consist of two pieces, as in the opercular 

 univalves (univalves with an operculum), and in most bivalves. 



A shell may consist of three pieces, as in the Terebratula 

 (Jigs. 112 and 113, page 89). 



A shell may consist of four or more pieces, as in some of the 

 Pholades. 



A shell may consist of many pieces, in which case it consti- 

 tutes a multivalve, as the Chiton (fg. 79). 



30. What are the characters of the Botryllus. (Botryllus, from the 

 Greek, botrus, a bunch of grapes.) 



31. What are the characters of the genus Pyrosoma ? (Pyrosoma, from 

 the Greek, pur, fire, and sower, body fire-bodies ; so called from possessing 

 the faculty of emitting light.) 



