CLASSIFICATION 35 



plete spongy network (Plate IV., Fig. 5, C). The 

 Oysters, freshwater Mussels, Cockles, Venus Shells, 

 Myas, and Ship-worms are examples of this order. 



Order IV. : Septibranchia (Plate XX., Figs. 

 20-23), in which the gill-filaments are completely 

 fused and transformed into a continuous muscular 

 septum, with several perforations to admit of the 

 circulation of the water (Plate IV., Fig. 5, D), A few 

 deep-water forms (Poromyiidse, Cetoconchidse, and 

 Cuspidariidae) constitute this order. 



Class V. : The CEPHALOPODA (Plate XXL), 

 of which the Nautilus, the Cuttlefishes, and 

 Octopods are examples, includes some of the most 

 highly organized of the Mollusca, as well as the 

 largest, for certain of the Cuttlefishes, it is calcu- 

 lated, exceed 50 feet in length. 



The Cephalopoda are symmetrical animals, the 

 two halves of the body corresponding in structure. 



The head, on either side of which there is a Large 

 and well-developed eye, is more or less distinct, and 

 is surrounded by the foot, which has, so to speak, 

 grown round it (Plate XXL, Fig. 1). In the Nautilus 

 the margins of the foot are divided into lobes, each 

 bearing a group of tentacles furnished with suctorial 

 ridges, that are retractile within special sheaths 

 (Plate XXL, Fig. 2). In the other groups the pedal 

 appendages take the form of four or five pairs of 

 elongate "arms," these arms being furnished with 

 rows of suckers or of hooks. 



