filO EMBRYONIC DEVELOPMENT OF MOLLUSKS. 



Teeth being very strongly brought-out by it (Fig. 321), and a 

 gorgeous play of colours being exhibited when a Selenite plate is 

 placed behind the object, and the analyzing prism is made to 

 rotate. * 



475. Development of Mollusca. — Although no application of the 

 Microscope is more important to the scientific Physiologist than 

 that which enables him to watch the successive steps of the process 

 of the Development of organized structures, yet the ordinary 

 Microscopist cannot be expected to feel the same interest in its 

 history, and will expect only to have his attention directed to such 

 of its phenomena as are of most general interest. The study of 

 the early stages of the Embryonic Development of Bivalve MoHusks 

 is attended with considerable difficulty, and has been, with few 

 exceptions, but very incompletely prosecuted. Of the very unsatis- 

 factory nature of our present knowledge of its history, we have 

 a marked example in the fact that what are undoubtedly the 

 Embryoes of a fresh-water Mussel, the A nodon cygneus,v> T hen found 

 adhering to the gills of their parent, have been described as 

 Parasites, under the name of Glochidium, and are still maintained 

 to be such by some persons who assume to be authorities on the 

 subject. It has been lately shown f that these Embryoes, after 

 being excluded from between the valves of their parent, attach 

 themselves in a peculiar manner to the fins and gills of small Fishes 

 (Fig. 322, a). In this stage of the existence of the young Anodon, 

 its valves are provided with curious barbed or serrated hooks (d, b), 

 and are continually snapping together (so as to remind the ob- 

 server of the Aricularia of Polyzoa, § 451), until they have 

 inserted their hooks into the skin of the Fish, which seems so to 

 retain the barbs as to prevent the re-opening 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, or, d) which 

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

 membrane, 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 

 branchiae and of oral tentacles ; but their nature can only be cer- 

 tainly determined by further observation, which is rendered diffi- 

 cult by the opacity of the valves. By keeping an adequate supply 

 of Fish, however, with these embryoes attached, any dexterous 

 Microscopist may overcome this difficulty, and may work out the 



* For additional details on the organization of the Tongue 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. 



t See the Rev. W. Houghton ' On the Parasitic Nature of the Fry of 

 the Anodonta cygnea,' in " Quart. Journ. of Microsc. Sci.," N.S., Vol. ii. 

 (1861), p. 162. 



