27 



irregularities that obstruct their progress of growth ; they are, however, 

 constantly liable to distortion, disease of the calcifying glands, and all " the 

 numerous ills that flesh is heir to." 



The sexes in the highest order of Gastropods, (Pectinibranchiata) are 

 separate, male and female ; the remainder are hermaphrodite, both sexes 

 combined in the same individual. The oviparous species have various modes 

 of producing their young. The Whelk {Buccinum undatmri) deposits her 

 eggs in masses of thin bladder-like capsules ; the Turbinella in a long cham- 

 bered nidus, in each compartment of winch are from twenty to thirty em- 

 bryos completely calcified ; and the Ianthina encloses her ova in a delicate 

 film of albumen attached to her curious float.* In the early development 

 of the Aplj/sia the shell is of a distinctly turbinated form, containing the 

 embryo closed in by an operculum ; a curious metamorphosis then ensues ; 

 the shell assumes an entirely internal position, winch in the adult forms 

 little more than a protective shield over the branchiae, a flattened or slightly 

 convex horny plate, with only the remotest trace of its primitive nucleus. 

 A somewhat similar phenomenon has also been noticed in the Tritonia and 

 Doris, which, though destitute of any horny or testaceous parts, either ex- 

 ternal or internal, in the adult state, are provided in an early stage of their 

 development with a delicate little horny nautiloid shell. The eggs of the 

 fresh-water species mostly consist of a transparent mucusf; and of the ter- 

 restrial species, the Helices deposit their eggs in the earth, whilst the arbo- 

 real Bulimi cement together a little nest of leaves for their beautifully 

 white eggs, which are sometimes nearly as large as those of a pigeon. 



In taking a general review of the habits of the Gastropods, many inte- 

 resting phenomena present themselves to our contemplation. The greater 

 portion of them live attached to masses of stone, in concealed situations, 

 such as the under surfaces, and in cavities and crevices ; also in coral sand 

 and mud, not, however, imbedded in such a state of torpor generally as the 

 acephalous or head-less tribes (the Bivalves), unless in the curious instance 

 of the Magilus. It is the peculiar property of this mollusk to become the 



* To this float the parent Ianthina commits her little progeny, and having securely fastened 

 their several cradles or nursery cells, she detaches the float, which bears the ova to the surface, 

 and sustains them where they may best receive the full influence of solar light and heat. — Owen. 



f In relating some experiments made by M. Jacquemin on the development of the Planorbis, 

 Professor Owen continues, " The rudiments of the head and foot are sufficiently obvious on the sixth 

 day ; the respiratory organs are formed on the sixth or eight day, according to the warmth of the 

 weather. Ou the eighth day the characteristic tentacles begin to sprout from the rudimental head. 

 On the tenth day all that part of the vitellus or embryo which is not occupied by the head, the 

 foot, and the breathing organ, is covered by a thin and transparent pellicle, which is the rudiment 

 of the shell. On the eleventh day one of the large central globules of the yolk begins to distin- 

 guish itself from the alimentary mass by feeble contractions and dilatations, of which about sixty 

 may be counted in a minute ; this is the heart. The mouth can now be discerned, and the small 

 eye-specks appear like black granules at the base of the tentacula. On the twelfth day the 

 embryo moves by its own contractions independently of the rotation produced by the cilia. On 

 the thirteenth day acts of deglutition are discernible ; the embryo swallows the remaining albu- 

 men, the anus is completed, and the genital organs begin to be formed. On the fourteenth day 

 the young Planorbis ruptures by more violent contractions the chorion, and escapes into the 

 water, protected by its own flexible shell." — Hunterian Lectures, 1844. 



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