MOLLUSCS. 363 



ribbon bearing seven rows of teeth, one rachidian, flanked on either side by three lat- 

 erals. The shells are ornamented with varices, the character of which serves to dis- 

 tinguish the two principal genera. In Manilla a varix is formed at 

 each half turn, so that the shell bears a flattened appearance, due to 

 the prominent and continuous varices of the whorls ; in Tritonium 

 the varices are not continuous, but are found at irregular intervals. 

 Almost all of the hundred and fifty species live between the tropics, 

 where they feed upon decaying animal matter, the species of Manella 

 being among the most prominent of the molluscan scavengers. The 

 species of both genera are handsomely colored, and some of those of 

 Tritonium are among the largest of gasteropods. The shells are 

 heavy, but the animals are, nevertheless, very active; some are lit- fig. 462.— ieoneJto 



/ . •' ' bituberculans. 



tora],'wnile others live m deeper water. 



Oedeb VI. — HETEROPODA. 



The naturalist, when skimming the surface of the ocean far from land, usually 

 captures numbers of pelagic forms with bodies as transparent as glass. When care- 

 fully studied, these forms are of the greatest interest ; there will be the curious worms 

 known as Sagitta, eel-like fishes {JLeptocephalus) whose position is far from certain, 

 shrimps and larvae of Crustacea, SalpcB, and jelly-fishes. Among the number will 

 usually be found specimens of the groups of molluscs known as heteropods and 

 pteropods. With the former of these two we have now to deal. The Heteropods 

 are streptoneurous gasteropods specially organized for a pelagic life, and the modifica- 

 tions which they have undergone serve to place them as highest in the series. Yet 

 small as is the group, the variations existing in it are by no means inconsiderable. 

 The foot has become modified for swimming, and may have three typical divisions 

 well marked, or it may be reduced to a thin vertical fin, small in proportion to the 

 body. The body may be developed in the normal manner, and the visceral hump 

 coiled in a spiral, bearing a shell, or it may be simply cylindrical and the visceral 

 hump completely atrophied. 



The nervous system is constructed on the usual gasteropod type, and, together 

 with the sense organs, is highly developed. The eyes, two in number, are enclosed in 

 separate capsules, and are moved by appropriate muscles. They are highly organized, 

 with cornea, iris, lens, and retina, and, like those of the cephalopods to be described 

 farther on, they are much like those of vertebrates, the principal difference being in 

 the relative positions of the fibres of the optic nerve and the layer of rods and cones. 

 The large ears are placed at or near the sides of the cerebral ganglion, and receive 

 their nervous supply from it. The organ of smell is also highly developed, and is 

 placed near the gills when these latter are present, or in a corresponding position 

 when the branchiae have disappeared. The organ consists of a groove of ciliated sen- 

 sory epithelium placed just above an olfactory ganglion and innervated in the normal 

 manner. 



Owing to the great transparency of the living animals, the study of the internal 

 structure is an easy task ; no dissection is necessary to ascertain the principal features, 

 all that is required being to place the animal under the microscope. The adjoining 

 figure of the structure of Atlanta was originally made in this way. The head extends 

 forward, more or less like a proboscis, and bears the eyes and usually a pair of ten- 

 voL. I. — 23 



