Survey of the Animal Kingdom 309 



in the same individual (hermaphroditic or monecious). There is no trochophore 

 larval stage in its embryologic development. 

 Example: Earthworm (Figs. 185 to 190). 



Class 3 — Hirudinea (hir u -din' e a) (Gr. Hirudo, leech). — Leeches have 

 dorsoventrally flattened bodies with anterior and posterior suckers for attachment 

 and blood sucking. There are no setae or parapodia. The real and visible seg- 

 ments have from two to fourteen external grooves to each real segment (depend- 

 ing on the species). The coelom may be small because of the growth of 

 mesenchyme cells. Both sexes are in the same individual. 

 Example: Leeches (Fig. 114). 



Phylum 10. Mollusca (mol -lus' ka) (L. ynollis, soft) 



General Characteristics 



Mollusks have soft bodies with no true skeleton, although many types secrete 

 one or more external calcareous shells from a fold of the body wall. These 



dorsal 



Ventral 



Fig. 115. — Chiton {Katharina sp.) of the class Amphineura, phylum Mollusca. 



animals are nonmetameric (unsegmented) -and possess either bilateral symmetry 

 or asymmetry, depending on the species. The animals are triploblastic, having 

 three primary germ layers: ectoderm, mesoderm, and entoderm. The coelom 

 (body cavity) is secondarily obliterated and is divided into a pericardial cavity 

 (around the heart) and a cavity around the reproductive organs. Mollusks 

 possess a mantle cavity between the main body and the mantle (enclosing en- 

 velope). A ventral muscular foot for locomotion is usually characteristic. Num- 

 ber of species of Mollusca, 75,000. 



Classification of the Phylum Mollusca 



Class 1 — Amphineura (am-fi-nu' ra) (Gr. amphi, on both sides; neura, 

 nerves). — These forms are widely distributed marine types which possess bilateral 

 symmetry with two nerves running the length of the animals. They often have 



