534 Mollusks. 



the ova placed in them afterwards, and then sealed by the adult an 

 mal. This mistake must have arisen from seeing the upper portion of 

 the case embraced by the animal while in the act of deposition ; which 

 may very commonly be seen. The ova are formed in the ovaries and 

 descend altogether into the oviduct. In passing through this long 

 and tortuous tube, they get enveloped in an albuminous-looking fluid, 

 and as they still further descend, become finally enclosed in a more 

 solid and horny secretion. This last forms the outside capsules, which 

 are deposited entire, enclosing the ova. The whole process of the de- 

 scent and formation of the capsules, is precisely similar to the forma- 

 tion of the egg-cases in the rays and sharks. 



When the cases are first deposited, they are semi-opaque or 

 milky, and their contents appear to be a thick tenacious fluid, of a 

 yellow colour. This, when subjected to the microscope, proves to 

 be granular; the grains are yellow, round, very small, opaque and sur- 

 rounded with a tenacious transparent fluid, similar in appearance to 

 the white of an egg : these constitute the ova. Immediately after de- 

 position they appear to be simple granules ; but soon, and without 

 much increase in size or alteration in shape, one spot on their surfa- 

 ces becomes covered with numerous vibratory cilia, which are in con- 

 stant and rapid activity. As the grains increase in size they become 

 oval, the cilia more distinct, and the spot on which they are situated, 

 as well as occasionally several other spots, become ti'ansparent ; thus 

 showing that the yellow yolk is enclosed in an investing membrane. 

 As development advances, this investure becomes more and more ap- 

 parent, especially towards that point where the vibratory cilia are si- 

 tuated. At this point, from the cilia, which are situated without, to 

 the yelk, situated within, several dark lines pass nearly parallel to each 

 other ; these afterwards prove to be the walls of a tube, w T hich forms 

 a communication between the albuminous envelope and yelk. This 

 external aperture to the tube varies in size and shape in different spe- 

 cimens, but is most commonly oval, and around its edges the cilia are 

 situated. The office most commonly assigned to cilia is that of re- 

 spiration ; and that probably is the function performed here : but the 

 vibrations are performed in the thick slimy fluid in which the ova lie ; 

 and as the embryo advances in development it disappears, so that 

 when the shells are ready to escape from the " cup," not a vestige of 

 it remains. As soon as the cilia appear, the yelk or rather embryo has 

 a gentle, circular, gliding motion, which is probably produced by the 

 action of cilia, but the cause is however rather obscure. The motion 

 is rather irregular; sometimes they glide forward, at others backward: 



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