CHAPTER XVI 
PHYLUM MOLLUSCA 
Classes :—1. GASTEROPODA, ¢.g. Snails, 2. SOLENOGASTRES—A small 
class of doubtful worm-like forms, e.g. Meomcenia, 3. SCAPHO- 
PoDA—A small class, e.g. Dentalium. 4. LAMELLIBRANCHIATA 
—Bivalves. 5. CEPHALOPODA—Cuttle-fiskes. 
THE series of Molluscs is in many ways contrasted with 
that of Arthropods; thus the body of the Mollusc is un- 
segmented, and there are no appendages. The general 
habit of life is also very different, for, although there are 
active Molluscs and sluggish Arthropods, it is true as an 
average statement that Molluscs are sluggish and Arthro- 
pods are active. In the frequent presence of a trochosphere 
larva, in the nerve-ring around the gullet, and in some other 
features, Molluscs resemble Annelids, but it is probable 
that they took their origin from a still lower level. 
GENERAL CHARACTERS 
Molluscs are unsegmented and without appendages. The 
symmetry ts fundamentally bilateral, but this is lost in most 
Gasteropods. The “foot”—a muscular protrusion of the 
ventral surface—ts very characteristic , it usually serves for 
locomotion, but is much modified according to habit. Typically, 
a projecting dorsal fold of the body-wall forms a mantle, or 
pallium (Fig. 205, ¢.), which often secretes a single or bilobed 
shell covering the viscera, and roofs in a space—the mantle 
cavity—within which lie the gills. But both mantle and shell 
may be absent. There are three chief pairs of ganglia—cere- 
brals, pedals, and pleurals—with connecting circum-esophageal 
commissures, and there ts also a visceral nervous system con- 
