MOLLUSCOUS ANIMALS GENERALLY. 183 



Mollusks that serve as their prey; this they are enabled to effect by evert- 

 ing that part of the probosis-shaped mouth whose floor is formed by the 

 flattened part of the tube, which is thus brought to the exterior, and. 

 by giving a kind of sawing-motion to the organ by means of the alter- 

 nate action of two pairs of muscles, a protractor, and a retractor, 

 which put-forth and draw-back a pair of cartilages whereon the tongue 

 is supported, and also elevate and depress its teeth. Of the use of the 

 long blind tubular part of the palate in these Gasteropods, however, 

 scarcely any probable guess can be made; unless it be a sort of * cavity 

 of reserve/ from which a new toothed surface may be continually sup- 

 plied as the old one is worn- away, somewhat as the front teeth of the Ro- 

 dents are constantly being regenerated from the surface of the pulps 

 which occupy their hollow conical bases, as fast as they are rubbed-down 

 at their edges. 



579. The preparation of these Palates for the Microscope can, of 

 course, be only accomplished by carefully dissecting them from their 

 attachments within the head; and it will be also necessary to remove the 

 membrane that forms the sheath of the tube, when this is thick enough 

 to interfere with its transparence. The tube itself should be slit up 

 with a pair of fine scissors through its entire length; and should be so 

 opened out, that its expanded surface may be a continuation of that 

 which forms the floor of the mouth. The mode of mounting it will de- 

 pend upon the manner in which it is to be viewed. For the ordinary 

 purposes of Microscopic examination, no method is so good as mounting 

 in fluid; either weak Spirit or Goadby's solution answering very well. 

 But many of these palates, especially those of the marine Gasteropods, be- 

 come most beautiful objects for the Polariscope when they are mounted 

 in Canada balsam; the form and arrangement of the teeth being very 

 strongly brought-out by it (Fig. 401), and a gorgeous play of colors being 

 exhibited when a selenite plate is placed behind the object, and the 

 analyzing prism is made to rotate. 1 



580. Development of Mollusks. Leaving to the scientific Embryologist 

 the large field of study that lies open to him in this direction, 2 the ordi- 

 nary Microscopist will find much to interest him in the observation of cer- 

 tain special phenomena of which a general account will be here given. At- 

 tached to the gills of fresh-water Mussels ( Unio and Anodon) there are 

 often found minute bodies, which, when first observed, were described as 

 parasites, under the name of Olocliidia, but are now known to be their 

 own progeny in an early phase of development. When a Fish is near, 

 they are expelled from between the valves of their parent, and attach 

 themselves in a peculiar manner to its fins and gills (Fig. 402, A). In 

 this stage of the existence of the young Anodon, its valves are provided 

 with curious barbed or serrated hooks (D, ), and are continually snap- 

 ping together (so as to remind the observer of the avicularia of Polyzoa, 

 554), until they have inserted their hooks into the skin of the Fish, 

 which seems so to retain the barbs as to prevent the reopening of the 

 valves. In this stage of its existence no internal organ is definitely 

 formed, except the strong 'adductor muscle* (c, a) which draws the 

 valves together, and the long, slender, byssus-filament (B, , D) which 

 makes its appearance while the embryo is still within the egg membrane, 



1 For additional details on the organization of the Palate and Teeth of the 

 Gasteropod Mollusks, see Mr. W. Thomson, in "Cyclop, of Anat. and Physiol. '" 

 Vol. iv., pp. 1142, 1143; and in "Ann. of Nat. Hist.," Ser. 2 Vol. vii., p. 86. 



8 See Balfour's " Comparative Embryology," Chap. ix. 



