DKECIOUS MOLLUSCA. 351 



and female to outward appearance ; the second evidently 

 marked by peculiarities in the structure of each sex. The 

 first may be called the cryptogamous Mollusca, and the second 

 tlie phaenogamous, for the terms mark very well the distinc- 

 tion between them. 



The cryptogamous Mollusca embrace the Tubulibranches, 

 the Scutibranches, and the Cyclobranches, which together 

 form but an inconsiderable proportion of the Gasteropods, for 

 they are orders few in species ; nor are the equally crypto- 

 gamic Heteropods more numerous in their tribes. They 

 were until recently considered by our best authorities to 

 be monoecious — a very natural error, for the proof of their 

 being bisexual is derived entirely from microscopic anatomy, 

 and is only to be found at certain seasons ; the truth had, 

 however, been suspected long before modern discovery pro- 

 claimed it decidedly. Adanson said that the limpets had 

 separate sexes ; * and Cuvier considered the opinion to be 

 probable, and perhaps applicable to the genera Capulus, 

 Crepidula, and Calyptrasa. f Mr. J. E. Gray convinced 

 himself of the fact by personal observation in all these 

 genera before others had given their assent ; ^ and now there 

 seems to be no dispute in the matter. The female deposits 

 her numerous ova enveloped in a jelly on its expulsion from 

 the body, and in their evolution, these ova appear to exhibit 

 no remarkable phases. 



The phasnogamic dioecious Mollusca embrace all the pec- 

 tinibranchial Gasteropods. You may remember being told 

 that these are either phytivorous or zoophagous, and these 

 tribes differ very remarkably in the mode of their genera- 

 tion. 



The Phytophaga lay their eggs merely enveloped in a 

 mass of jelly, just firm enough to retain its form in the 

 water, and which, deposited on the fronds of sea-weed, or 

 on the surface of rocks and stones, adheres to them with 

 tenacity. § The form of the mass is roundish, oval, or ob- 

 long, and it may be more complex in some. 1 have often 

 seen, on the common wreck of our shores, a white gela- 

 tinous ring, with a narrow break or interruption on one 

 side, which I believe to be the spawn of the Trochus cine- 

 rarius. The ova are always immersed in the mass which 

 forms a common bed to the whole ; but besides each ovum, 



* Hist. Nat. du Senegal, 31. 

 + Mem. xviii. 18, 20, 21 and 22. 

 + Edinb. Journ. Nat. and Geogr. Sc. iii. 53. 



§ Baster has given rude figures of the spawn of our common Jjittorinaj 

 in Opusc. Subs. 1, pi. 5. fig. 4, 5. 



