404 PHYLUM MOLLUSC A. 



and in some other forms, there is a struggle for existence 

 -an infant cannibalism in the cradle, for out of the 

 numerous embryos in each capsule only a few reach 

 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. 201.) 



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 Palaeozoic 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 II. SOLENOGASTRES. 



The members of this class are worm-like animals, in which the 

 mantle envelops the whole body and bears numerous spicules, but no 

 shell. It is somewhat doubtful if they are Molluscs at all. There are 

 two families Neomeniidoe and Ch?etodermidae. 



Of Neomeniidse, six genera are known, e.g. Neomenia and Pro- 

 neomenia. They have a longitudinal pedal groove, an intestine without 

 distinct digestive gland, two nephridia with a common aperture, and 

 hermaphrodite reproductive organs. The Choetodermidae, represented 

 by one genus Chcctoderma, are cylindrical in form, without a pedal 

 groove, with a radula bearing one tooth, with a distinct digestive gland, 

 and with two nephridia opening separately into a posterior cavity, which 

 also contains two gills. The sexes are separate. 



