DEVELOPMENT OF MOLLUSC A 



933 



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 depend upon, the 

 manner in which it is to be viewed. For 

 the ordinary purposes of microscopic ex- 

 amination no method is so good as mount- 

 ing in fluid, either weak spirit or Goadby's 

 solution answering very well. But many 

 of these palates, especially those of the 

 marine Gastropods, become 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. 709), and 

 a gorgeous play of colours being exhibited 

 when a selenite plate is placed behind the F 

 object, and the analysing prism is made to 

 rotate. 1 



Development of Molluscs. Leaving to 



the scientific embryologist the large field of study that lies open to 

 him in this direction, 2 the ordinary microscopist will find much to 

 interest him in the observation of certain special phenomena of 



709. Palate of Buccl- 

 num undattun as seen under 

 polarised light. 



which a general account will be here given. 



Attached to the gills of 



fresh-water mussels (Unto and Anodon) there are often found in the 

 spring or early summer minute bodies which, when first observed, 

 were described as parasites, under the name of (.ilnclii,li,i, Inir are 

 now known to be their own progeny in an early phase of develop- 

 ment. When they are expelled from between the valves of their 

 parent, they attach themselves in a peculiar manner to the fins and 

 gills of fresh-water fish. In this stage of the existence of the young 

 Anodon, its valves are provided with curious barbed or serrated 

 hooks (fig. 710, A), and are continually snapping together, 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 (a ad) which draws the valves 

 together, and the long, slender byssus-filament (by) which makes 

 its appearance while the embryo is still within the egg-mem- 

 brane, lying coiled up between the lateral lobes. The hollow of 

 each valve is filled with a soft granular-looking mass, in which 

 are to be distinguished what are perhaps the rudiments of the 



1 For additional details on the organisation of the palate and teeth of the 

 Gastropod molluscs, see Mr. W. Thomson in Cyrluji. Aunt, and Pltysiol. vol. iv. 

 pp. 1142, 1148, and in Ann. Xnt. Hist. ser. ii. vol. vii. p. 86; Professor Troschel, Da* 

 Gebiss der SchnecJcen, Berlin, 1856-79; A. Riicker, ' Ueber die Bildung der Kadula 

 bei Helix jjomt/tia,' Bericlit oberhess. Gesellscli. Giessen, xxii. p. 209 ; P. Geddes, ' On 

 the Mechanism of the Odontophore in certain Molluscs,' Trans. Zb'ol. Soc. x. p. 485. 



2 See Balfour's Comparative Embryology, vol. i. chap. ix. More recent text- 

 books of embryology, such as that of Professor Korschelt and Heider, need not here 

 be specifically cited. 



