DIOECIOUS MOLLUSCA. 353 



nation is sufficient for at least two generations, as we know 

 to be the case with the Aphides or Plant-lice.* 



The zoophagous Gasteropods, living usually in deeper 

 water, where the spawn is more exposed to forage, enclose 

 their eggs in capsules of a horny texture, and often so curi- 

 ously connected and contrived that nowhere will you find 

 finer displays of the Creator's preserving care over all his 

 works. The ova, while passing from the body, and while 

 still in the oviduct, are included within a glairy excretion 

 prepared in an accessory glandular apparatus, and which 

 is moulded into a coriaceous pouch of very variable figure 

 according to the species. It is either single or compound. 

 In the former case, every pouch, as it is extruded, is at- 

 tached by the animal, one by one, to the rock chosen for 

 the precious trust ; in the latter case, the cluster of oviferous 

 receptacles is expelled in one common mass : and when you 

 are told that this is frequently much larger than the shell 

 of the parent, you may wonder how this can be ? And so 

 did Dr. Job Baster. " I have often wondered," he says, 

 " how a univalve could lay an ovary exceeding, by five or 

 six times, its own size. But the explanation is obvious 

 enough, for it is proportionably small, and of a soft gluti- 

 nous consistence on its first deposition; it grows after its 

 expulsion, and keeps pace with the growth of the ova and 

 young, and at the same time hardens to a more solid and 

 coriaceous texture. I have seen the receptacles of the 

 Purpura lapillus when they were less than a line in height, 

 and their colour was also then much darker than it is at 

 maturity." The process of laying is very well described by 

 a correspondent of Sir E. Home's as observed by him in the 

 Turbinella pyrum. " A friend of mine," says Sir Everard, 

 " saw the female shed her eggs ; a mass, apparently of mucus, 

 passed along the deep groove in the lip of the shell in the 

 form of a rope, several inches in length, and sunk to the 

 bottom : this rope of eggs, enclosed in mucus at the end 

 last discharged, was of so adhesive a nature, that it became 

 attached to the rock, or stone, on which the animal depo- 

 sited it. As soon as the mucus came in contact with salt 

 water, it coagulated into a firm membranous structure, so 

 that the eggs became enclosed in membranous chambers, and 

 this connected nidus, having one end fixed and the other 



* M. Alex, de Nordmann found that some Nudibranchial genera laid 

 fruitful ova without the individual having had any society with another of 

 its species. Ann. des Sc. Nat. (1846) v. 136. Being of androgynous 

 structure, this fact is more readily explained than it is in Paludina. See 

 Alder and Hancock's Nudibr. Mollusca, iii. fam. 3. pi. 7 and 8. 



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