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. 
; : page 
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 (Poromyiide, Cetoconchide, and 
Cuspidariidz) constitute this order. 
Crass V.: The CEPHALOPODA (Plate XXI.), 
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 XXI., Fig. 1). Inthe 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 XXI., 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. 
