. Outlines of zoology. Zoology. 36o MOLLUSC A. shore and the sea bottom at relatively slight depths ; the deep sea forms are comparatively few. Gasteropods rarely feed at such a low level as bivalves do, indeed some of them are fond of eating bivalves. Most prosobranchs (streptoneural), with a respiratory siphon and a shell notch in which this lies, are carnivorous, , the buckies {Buccininn) and " dog whelks " {Purpura); on the other hand, those without this siphon, and with an un- notched shell mouth, feed on plants, , the seaweed eating periwinkles {Littorina). The vege- ta
. Outlines of zoology. Zoology. 36o MOLLUSC A. shore and the sea bottom at relatively slight depths ; the deep sea forms are comparatively few. Gasteropods rarely feed at such a low level as bivalves do, indeed some of them are fond of eating bivalves. Most prosobranchs (streptoneural), with a respiratory siphon and a shell notch in which this lies, are carnivorous, , the buckies {Buccininn) and " dog whelks " {Purpura); on the other hand, those without this siphon, and with an un- notched shell mouth, feed on plants, , the seaweed eating periwinkles {Littorina). The vege- tarian habits of most land snails and slugs are known to all. Many Gasteropods, both marine and ter- restrial, are very voracious and in- discriminate in their meals; others are as markedly specialists or epi- cures. Some marine forms, partial to Echinoderms, have got over the difficulty of eating such hard food by secreting dilute sulphuric acid, which is said to change the car- bonate of lime in the starfish into the more brittle and readily pul- verised sulphate. Only a few Gas- teropods are parasitic, , Euliina and Stylifer on Echinoderms, and the extremely degenerate Efitoconcha inirabilis,—within the Holothurian Synapta. Life History. The eggs of Gasteropods are usually small, without much yolk, but surrounded by a jelly, the sur- face of which hardens. In the snail and in some others there is an egg shell of lime. Sexual union occurs between her- maphrodites as well as between separate sexes, and ferti- lisation is effected inside the genital duct. Development sometimes proceeds within the parent, but in most cases. Fig. 117.—Stages in Molluscan development. .4, Blastula of Limpet (after Patten); B, Gastrulaof/'o/w- (//war'/V'z^^izra (after Tonniges): z\ beginning of velum ; arc, archenteron ; ?«, mesoderm cells; C, later stage of the same; z\ velum ; tn, mouth invagination ; arc. archen- teron : a, anus ; /, beginning of foot ; , shell Please note tha
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
