Phylum MoUusca and Mi}wr Eucoelomate Phyla 467 



tion. All the organ systems are present and well developed. They 

 possess other characteristics in common with the higher invertebrates, 

 but many of them are highly modified. 



All molluscs, for example, are bilaterally symmetrical, but in some 

 groups the visceral organs have been twisted into an almost asymmetri- 

 cal pattern. The coiled shell obscures the fundamental bilaterality of the 

 snail. A true coelom is present in all molluscs, but its degree of de- 

 velopment varies in the different classes. In most forms, it is reduced 

 to the pericardial cavity surrounding the heart and spaces within the 

 kidneys and gonads which are connected with the pericardial cavity. 



In these forms, the visceral organs are grouped as a well-defined 

 mass, "the visceral mass." 



Among some molluscs, cephalization is well developed. There is 

 a head region with concentrations of nervous tissue and highly developed 

 sense organs among the snails, squids, and octopuses ; in the clam, 

 cephalization is practically nonexistent. 



Unlike the annelids and arthropods, the molluscs lack segmentation. 

 While segmentation is of great value for movement in the arthropods 

 and annelids, it would be of but little value to a body form such as 

 that developed by the molluscs. For the shelled forms, a muscular 

 foot serves as the locomotor apparatus, while the squids and octopuses 

 have, in general, flexible bodies. In all classes except that of the squids 

 and octopuses, the foot is well developed and variously shaped. Among 

 the latter it is modified into long tentacles possessing suckers. These, 

 too, are used for crawling and grasping, although very rapid movements 

 are accomplished by the ejection of water from the mantle cavity. 



The glandular mantle is a unique possession of the members of this 

 phylum. In those molluscs possessing an external shell, it is located 

 directly under the shell ; in the others its position is variable. In all, 

 however, the mantle does secrete the shell whether it be an internal or 

 external one. 



There is a complete digestive tract which is again variously modi- 

 fied in the different classes. Among the snails, it is often U-shaped with 

 the mouth and anus opening close together ; in clams it is coiled, but 

 the anus opens at the posterior end of the body. There is a large di- 

 gestive gland associated with the tract. In most forms there is a spe- 

 cial rasping organ, the radiila, in the mouth. 



The circulatory system is of the open type, that is, there is a 

 heart consisting of a ventricle and auricle, blood vessels leading into 



