552 PTEEOPODA. 



(1475.) From the ninth to the eleventh day the ciliated disks become 

 more developed, more separated from each other, and more moveable ; 

 the largest of the four lobes of the body has arranged itself into stomach 

 and intestine, in which occasional contractile movements may be seen. 



(1476.) The case-membrane previous to the escape of the embryo 

 becomes gradually thinner, and at last either entirely disappears or is 

 reduced to shreds, probably by the incessant strokes upon its inner sur- 

 face of the long cilia of the ciliated disks during the active revolutions 

 of the embryo round its interior. The embryo at the time of its libera- 

 tion is provided with a shell somewhat resembling that of the Nautilus, 

 from which it can protrude the anterior part of its body and retract it 

 at pleasure (fig. 276, 6), swimming about actively in the surrounding 

 water by means of its ciliated disks. 



(1477.) The spawn of other Gasteropods is deposited under diverse 

 forms. In the marine species it is usually found attached to the surface 

 of stones, shells, or sea-weed, the ova being connected with each other 

 in long ribands or delicate festoons, which are sometimes extremely 

 beautiful and curious. The Doris and Tritonia deposit their ova in long 

 gelatinous bands, resembling beautiful frills of rich lace. In Aplysia 

 the spawn resembles long strings of jelly, in which the ova are seen, 

 varying in tint, so as to give different colours to different parts of the 

 thread. In Helix and Bulimus the eggs are protected by a hard shell ; 

 whilst in Buccinum, Valuta, Mure.v, and other marine species the ova 

 are enveloped in membranous capsules, agglomerated together in large 

 bunches. These capsules have been sometimes erroneously regarded as 

 the eggs themselves ; they are, however, merely coriaceous envelopes, 

 answering the purpose of the gelatinous coating that encloses the eggs 

 of other species. Many of the Gasteropoda are exceedingly prolific : a 

 single Doris will lay 50,000 eggs at a birth ; and when we take into 

 account that all the individuals are prolific (the sexes being combined), 

 and that each will produce spawn two or three times in a season, it is 

 evident how vast must be the number of their progeny. 



CHAPTER XXIII. 



PTEEOPODA* (Cuvier). 



(1478.) NEARLY allied to the Gasteropods in their internal organiza- 

 tion, but differing from them remarkably in the character and position 

 of their locomotive apparatus, are the PTEBOPODA a class of mollusks 

 of small dimensions, but met with in astonishing quantities, at certain 

 seasons, in various parts of the ocean. So numberless, indeed, are 

 * irTfpbv, a wing ; TTOVS, nodus, the foot. 



