124 Animal kingdom. 



without regular branchiae ; and unlike the Brachiate Mollusks in not 

 having a circle or spiral of ciliated tentacles, or having them only in 

 a rudimentary state. Also having a leathery or membranous exterior, 

 without a shell. 



III. Brachiate Mollusks. Without regular branchiae ; the 

 shells, when any exist, bivalve, but transverse across the back and 

 venter, instead of vertical either side of the body ; the head having a 

 fringe of slender organs arranged around the mouth, or in two spiral 

 groups either side of the mouth. These Mollusks, the earliest in 

 geological history, have some worm-like characteristics, as shown by 

 Morse ; but they are true Mollusks in wanting the multiplicate feature 

 of Articulates, as well as in other points. 



I. Ordinary Mollusks. 



The Ordinary Mollusks are divided into — 



(1.) The Acephals, or headless Mollusks, the head not being distinctly 

 defined in outline ; as the Oyster and Olam ; 



(2.) The Cephalates, having a defined head ; as the Snail ; and, 



(3.) The Cephalopods, having the head furnished with long arms 

 (or feet) ; as the Cuttle-fish. 



The Acephals have a mouth, but no perfect organs of sight ; the 

 Cephalates have distinct eyes and a distinct head (Fig. 156) ; the 

 Cephalopods have the eyes large, and can grasp with great power by 

 means of their arms, which are furnished with suckers (Fig. 159). 



The pallium starts from the back, and often covers the sides of the 

 body like a cloak, and is either open or closed along the venter : it is 

 also called a mantle. It lies against the shell in the oyster, clam and 

 allied species, and secretes it ; and, in some univalves or Gasteropods, 

 it may be extended out over more or less of the exterior of the 

 shell. 



1. Cephalopods, or Cuttle-fishes. — There are two orders of Ceph- 

 alopods ; one having external shells, and four gills or branchiae ; a 

 second, having sometimes internal shells but no external, and having 

 but two branchiae. The external shells are distinguished from those 

 of Gasteropods (or ordinary univalves) by having, with a rare excep- 

 tion, transverse partitions, — whence they are called chambered shells 

 (Fig. 158). They may be either straight, or coiled; but with few ex- 

 ceptions they are coiled in a plane, instead of being spiral. A tube, 

 called a siphuncle, passes through the partitions ; and this siphuncle 

 may either be central or nearly so, as in the genus Nautilus (Fig. 

 158, which represents a shell cut through the middle plane, so as 

 to show the partitions and the siphuncle), or lie along the inner or 

 ventral side of the cavity, or the outer or dorsal side, as in Ammonites. 



