CHAPTER 14 



The Phylum Mollusca 



104. General Features of the Molluscs 



The Mollusca, which inchides snails, clams, squids, and others, are a 

 group of sott-bodied animals that usually secrete external protective 

 shells. The ventral portion of the body is elaborated as a muscular 

 organ, called the foot, used in locomotion. Many of the molluscan groups 

 have in the mouth a unique rasping organ, the radula. These structures 

 will be described later. 



Molluscs have both a eucoelom and a circulatory system. The 

 coelom is small, and is associated with the heart, gonads and excretory 

 organs. The portion surrounding the heart, the pericardial cavity, is 

 the most obvious. 



The circulatory system is well developed. It is modified variously 

 in the different groups, but typically includes a single dorsal heart (Fig. 

 14.1) composed of one anterior ventricle and a pair of posterior auricles. 

 Tire auricles receive blood from veins and pump it into the ventricle 

 while the latter is relaxed. Then the ventricle, a heavily muscled organ, 

 pumps the blood out through arteries to all parts of the body. The 

 blood may pass through capillaries to the veins, but usually passes into 



Ventricle Auriclcz. 



Figure 14.1. A diagram showing the principal features of the molluscan circulatory 

 system. Auricles, gills and nephridia are usually paired. 



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