300 



COLLEGE ZOOLOGY 



The giant clam (Tridacna) of the tropics 

 may reach a length of three feet and weigh 

 500 pounds. The large shells are used as 

 cradles for babies by some natives of the 

 East Indies. 



Among the products of value derived from 

 mollusks are pearls and peari buttons. Pearis 

 are found especially in pead oysters {Pinc- 

 tada) in Ceylon, India, Japan, and north 

 Australia. They also occur in the common 

 oyster and in clams, but these are never of 

 high value. The shells of fresh-water mussels 

 are used for the manufacture of pearl but- 

 tons and many tons are collected annually 

 in the United States, mainly from the Mis- 

 sissippi and Ohio rivers and their tributaries. 

 Overfishing of mussels for buttons has seri- 

 ously depleted them. The U.S. Fish and 

 Wildlife Service has conducted many experi- 

 ments in an effort to improve mussel fishing. 



CLASSIFICATION OF 

 THE MOLLUSCA* 



[For reference purposes only) 



Mollusks are unsegmented invertebrates (ex- 

 cept the class Monoplacophora), and without 

 jointed appendages. They usually possess a 

 shell which is secreted by a mantle. A mus- 

 eular foot of some sort is generally present. 

 The classification of the more than 90,000 

 species is based on the characteristics of the 

 foot, mantle, shell, radula, and respiratory 

 organs. Six classes, three subclasses, and eight 

 orders are described here as follows: 



Class 1. Monoplacophora (bearing one flat 

 shell). This name was originally pro- 

 posed to cover a group of extinct 

 paleozoic mollusks, but now contains 

 a remarkable living form Neopilina 

 "(Fig. 180). A small disklike foot, a 

 single shell, but the body is divided 

 into segments with pairs of muscle 

 scars on the inner surface of the shell, 



* This classification is according to W.J. Clench 

 and R.D. Turner, Museum of Comparative Zoology, 

 Harvard University. 



and paired auricles, nephridia, and 

 breathing organs. This is the only 

 class of segmented mollusks. 

 Order 1. Tryblidiacea. Shell is spoon- or 

 cap-shaped. The animal has a 

 head region and 5 well-devel- 

 oped metamercs which ha\e 

 paired auricles, nephridia, and 

 comblike gills (ctenida). The 

 extraordinary deep-sea form, 

 Neopilina galatheae, first re- 

 ported in 1957, is the only 

 known, living, segmented mol- 

 lusk. 

 Class 2. Amphineura (Or. amphi, both; 

 neuron, nerve). Chitons (Fig. 170). 

 Marine; elongate body; head re- 

 duced; shell of 8 plates or none; no 

 tentacles; and bilateral symmetry 

 obvious. 

 Order 1. Polyplacophora. Chitons. Ellip- 

 tical body; large flat foot; shell 

 a middorsal row of 8 plates, 

 surrounded by a fleshy girdle; 

 6 to 80 pairs of gills in groove 

 around foot. Sexes separate. Exs. 

 Tonicella and Chiton. 



Order 2. Aplacophora. Wormlike forms; 

 integument thick; lacks any evi- 

 dence of shell, except tiny limy 

 spicules in the mantle; and 

 rudimentary foot. Ex. Neo- 

 menia. 

 Class 3. Scaphopoda (Gr. skaphe, boat; pons, 

 foot). Tooth shells or tusk shells 

 (Fig. 170). Marine; body enclosed 

 in tubular shell, open at both ends, 

 no gills, delicate tentacles, and dioe- 

 cious. Ex. Dentalium entale; from 

 Cape Cod northward. 

 Class 4. Pelecypoda (Gr. pelekys, hatchet; 

 pous, foot ) . ( Lamellibranchiata ) . 

 The pclecypods are familiar forms to 

 most people because of their eco- 

 nomic importance. Bivalve mollusks, 

 clams, mussels, oysters, scallops, nut 

 clams, cockles, shipworms, etc. (Figs. 

 170, 174, 187, 188). Marine or in 

 fresh water; mantle secretes shell, usu- 

 ally two valves; head, eyes, tentacles; 

 radula absent; foot usually wedge- 



