OTHER MOLLUSKS 131 



another? To which part is the operculum attached when present? 

 Anterior to the foot is the mouth. What is its shape? On each side 

 are fleshy tentacles. Determine their size and shape? Touch them 

 with a needle. What happens? Are there one or two pairs? Do you 

 find small glistening spots, the eyes, on the tentacles? What is their 

 position? Locate the anal opening on the right side anteriorly. In 

 the air-breathing, or pulmonate, snails find the respiratory opening 

 near the anal opening. Observe living snails in aquaria and notice 

 their locomotion and feeding activities. 



(c) If the internal structure is to be studied, large snails, such as 

 Busycon, will be furnished, and additional directions will be given. 



Exercise 2. — Other Representative Mollusks. 



(d) Refer to Fig. 74, p. 171, for classification within the Phylum 

 MoUusca. Examine figures or demonstration specimens as follows: 

 (1) Amphineura, such as Chiton, showing externally the segmented 

 shell, foot, gills, mouth, and anus. (2) Pelecypoda, among which the 

 small fresh-water Sphserium may be examined alive in a watch glass 

 and compared with the fresh-water mussel previously studied. It is 

 viviparous, and young ones ready to escape may be seen through the 

 shells of larger individuals. Marine clams and oysters, particularly 

 their shells, may be studied. The shells of the clam, salt-water mussel, 

 Mytilus, and oyster illustrate interesting types of modifications. In 

 Pecten, the animal is adapted for swimming and has eyes and tentacles 

 along the mantle margin. (3) Gastropoda have been studied alive as 

 represented by the pond snail. The shells of some marine snails are 

 modified in peculiar ways. If specimens are available, examine them 

 for lines of growth and shape and try to determine how specialized 

 types have been modified from the more usual forms. (4) Cephalopoda 

 are exclusively marine. In contrast to the sluggish activities of most 

 other mollusks, they are active free-living forms which capture fish 

 and such animals for food. Examine a squid, Loligo. What structures 

 are related to its active life? Compare the "head" with the corres- 

 ponding region of a clam, a snail, and a chiton. The squid's tentacles 

 are a modification of the foot. The shell is internal in the adult. 

 The eye is almost as complex as that of a vertebrate, although it 

 develops in a different manner. The Octopus, or devil-fish, is another 

 cephalopod; also the Sepia, or cuttle-fish, the shells of which are sold 

 as beak-sharpeners for canary birds. (5) The Scaphopoda, repre- 

 sented by Dentalium, show another type of specialization which may 

 be visualized by thinking of a chiton as elongated dorso-ventrally and 

 developing a conical foot. 



