Ventricle 



Anterior aorta 

 Stomach 



Anterior adductor 

 muscle 

 Cerebropleural 

 ganglion 



Mouth I ^^ 

 Pedal ganglion 

 Foot 

 Digestive gland 



Gonad 



Intestine 



Auricle 



Pericardial cavity 



Posterior adductor 

 muscle 

 Posterior aorta 

 . Anus 

 N^^Excurrent 



siphon 

 ^ Incurrent 

 ^i siphon 

 y Mantle 



Visceral gangUon 

 Nephridium 



Gill 



Fig. 13.1. Internal structure of a typical fresh-water pele- 

 cypod, drawn as though the left valve had been removed 

 and the body dissected from the left side; semidiagrammatic. 



Chordata. From the standpoint of numbers of species and individuals, as well 

 as of adaptability to various environmental situations, each of these climax 

 groups represents a markedly successful type of organization. It is fruitless 

 to speculate about which is "higher," an arthropod or a chordate, or about 

 which of these evolutionary lines has produced the more successful type 

 of animal. 



In this and subsequent chapters, we shall consider first the schizocoelous 

 phyla, beginning with the phylum Mollusca, and then the enterocoelous 

 animals. 



The Phylum Mollusca 



The Mollusca may be defined as bilateral, unsegmented, triploblastic 

 animals, typically with a calcareous exoskeleton. The body is divided into 

 head, foot, and visceral regions, and a single pair of compound excretory 

 organs is present. Except in certain specialized members of the phylum, the 

 body is enclosed by a characteristic outgrowth of the body wall, the mantle, 

 which is responsible for the secretion of the exoskeleton. The word Mollusca 

 is derived from the Latin molluscum, meaning soft, and refers to the texture of 

 the body within the shell. The phylum consists of five classes: the class 

 Amphineura, or chitons, typically flattened forms with segmented dorsal shell 

 plates; the class Gastropoda, represented by the snails and slugs; the class 

 Pelecypoda, or bivalve moUusks such as clams and mussels; the class Scapho- 



369 



