Elements of conchology Prepared Elements of conchology / Prepared for the use of schools and colleges elementsofconcho00rusc Year: 1844 m ROSOM A. 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. f. 115.—PYROSOMA. 31. The Pyrosomje unite in great numbers, forming a large hollow cylinder, open at one end and closed at
Elements of conchology Prepared Elements of conchology / Prepared for the use of schools and colleges elementsofconcho00rusc Year: 1844 m ROSOM A. 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. f. 115.—PYROSOMA. 31. The Pyrosomje 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 her^ 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 recog-nise 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 Terehratula {figs. 112 and 113, page 89). A shell may consist of four or more pieces, as in some of the Pliolades. A shell may consist of many pieces, in which case it consti- tutes a multivalve, as the Chit
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