166 INVERTEBRATE ANIMALS. 



long, ciliated arms, which spring from the sides of the mouth, 

 seem to be the main agents in respiration. In the bivalve- 

 shell-fish, the cuttle-fishes, and most of the univalves, the 

 breathing-organs are in the form of gills or branchise, adapted 

 for breathing air dissolved in water. In the remainder of the 

 univalves (e. g., snails and slugs), the breathing-organs are 

 adapted for breathing air directly, and have the form of an 

 air-chamber or pulmonary sac, produced by the folding of a 

 portion of the mantle. The air is admitted to the chamber by 

 a round opening, situated on the side of the neck, and capable 

 of being closed at will. The lining membrane of the chamber 

 is richly supplied with blood-vessels, and thus the necessary 

 purification of the blood is carried out. 



In accordance with the scattered or rudimentary condition 

 of the nervous system, the Mollusca are not characterized by 

 acuteness of senses, nor by any great power of locomotion. 

 Organs of sight exist in some of the lower and many of the 

 higher Mollusoa, attaining in the cuttle-fishes (Fig. 89) an ex- 

 tremely high type of organization. The common bivalve shell- 

 fish, such as the scallop, possess numerous simple eyes placed 

 along the margins of the mantle, but, in many cases, even these 

 are absent. Locomotion is very variously effected, but seldom 

 with much vigor or activity. The lowest classes of the Mol- 

 lusaa are, in the great majority of instances, fixed when adult. 

 The common univalve shell-fish, such as whelks, snails, slugs, 

 etc., creep about slowly by means of a flattened disk, devel- 

 oped on the under surface of the body, and known as the 

 "foot." Other Univalves and many Bivalves can eifect short 

 leaps by means of the foot, but many of the latter are perma- 

 nently fixed to solid objects, or buried in the sand. The mi- 

 nute Molhtsoa, known as the Pteropoda (Fig. 88), swim freely 

 at the surface of the ocean by means of two fins, formed by a 

 modification of the foot, and attached to the sides of the head. 

 The only MoUusks which enjoy really active powers of loco- 

 motion are the predacious cuttle-fishes, which swim rapidly by 

 means of fins, or by ejecting a jet of water from the cavity of 

 the mantle, and which can also creep about by means of the 

 " arms " placed around the mouth (Fig. 89). 



The last feature in the Mollusca which requires to be men- 

 tioned is the " shell." The shell is not invariably and univer- 

 sally present in the Mollusoa, many being either destitute of 

 a shell altogether, or having one so small that it would not com- 

 monly be recognized as such. In these cases, as in the com- 

 mon slugs, the animal is said to be " naked." In all the Mol- 



