SCAPHOPODA. 377 



maturity, — those that get the start eating the others as 

 they develop. 



The development is usually simple and typical. In other 

 words, segmentation is total though often unequal ; gastrula- 

 tion is embolic or epibolic according to the amount of yolk 

 present ; the gastrula becomes a trochosphere, and later a 

 veliger. (Fig. 166.) 



Past history. — As the earth has grown older the Gasteropods have 

 increased in numbers. A few have been disinterred from the Cambrian 

 rocks ; thence onwards they increase. Most of the Palseozoic genera 

 are now quite extinct, but many modern families trace their genealogy 

 to the Cretaceous period. Those with respiratory siphons were hardly, 

 if at all, represented in Palaeozoic ages, and the terrestrial air-breathers 

 are comparatively modern. 



Bionomics. — As voracious animals, with irresistible raspers, 

 Gasteropods commit many atrocities in the struggle for exist- 

 ence, and decimate many plants. Professor Stahl shows, 

 however, that there are more than a dozen different ways 

 in which plants are saved from snails,— by crystals, acids, 

 ferments, etc.; in short, by constitutional characteristics 

 sufficiently important to determine survival in the course of 

 natural selection or elimination. As food and bait, many 

 Gasteropods are very useful ; their shells have supplied tools 

 and utensils and objects of delight ; the juices of Purpura 

 and Murex furnished the Tyrian purple, more charming 

 than all aniline. 



Class III. SCAPHOPODA. 



Very different in many respects from Gasteropoda are the Scaphopoda, 

 of which Dentalium (Elephant's tooth-shell) is the commonest genus. 

 They are apparently related to the Zeugobranchiate Gasteropods, and 

 also to the simplest Bivalves. They burrow in the sand at considerable 

 depth off the coasts of many countries. The mantle has originally two 

 folds, which fuse ventrally, and the shell becomes cylindrical, like an 

 elephant's tusk. It is open at both ends. The larger opening (directed 

 downwards in the sand) is anterior, the concave side of the shell is 

 dorsal. The mouth opens at the end of a short buccal tube, at the base 

 of which is a circle of ciliated tentacles. The foot is long, with three 

 small terminal lobes. It is used in slow creeping, and is protruded at 

 the anterior opening. There are cerebral and pleural ganglia near one 

 another in the head, pedal ganglia in the foot, and a long untwisted 

 visceral loop with olfactory ganglia near the posterior anus. Sense 

 organs are represented by otocysts beside the pedal ganglia. There is 

 an odontophore with a simple radula. The food consists of minute 



