46 BIOLOGICAL LECTURES. 



All these parts are entirely functionless, and great only in 

 potentialities. 



The glochidium is a larva highly specialized for a peculiar 

 mode of development. When the larva is extruded into the 

 water by the mother-clam, it falls to the bottom and lies there 

 with its thread floating in the water, violently snapping its 

 valves together at the least disturbance. It is really a most 

 sensitive little creature, and so readily perceives any fish swim- 

 ming by. If it be so fortunate as to have its thread caught on 

 the fish, it at once attacks it with its hooks. It is, of course, 

 only on the unprotected parts of the fish, such as the fins or 

 the gills, that the larva is able to make any impression; and it 

 is stated that the larvae of Aiiodonta usually become parasitic on 

 the fins, and those of Unio on the gills. The irritation caused 

 by the presence of the larva sets up a proliferation of the epi- 

 dermis, which speedily encloses the little parasite in a sort of 

 cyst, within which the later development takes place. 



II. Cleavage of the Egg of Unio. 



Adaptation in cleavage can manifest itself only in the three 

 possible modes of cleavage variation, which are, as has been 

 pointed out by Mead, Jennings, zur Strassen, and others, first, 

 differences in the rate of cleavage of cells ; second, differences 

 in size ; and third, differences in the direction of the cleavage 

 or position of the resulting parts. Let us now examine the cleav- 

 age of the &^^ of Unio with these general principles and the struc- 

 ture of the larva in mind. I may as well state at once that I 

 regard the. general form of the cleavage as inherited from a long 

 series of ancestors, extending back, probably, to the Turbcllaria. 



The general plan of the cleavage is the same as in annelids, 

 Tnrbellaria, and most mollusks, described in other lectures con- 

 tained in this volume. From the first four cells, separated by 

 meridional planes, the ectoderm is separated by three slightly 

 oblique cleavages horizontal in their general direction, the first 

 being dexiotropic, the second l?eotropic, and the third dexiotropic 

 again. A fourth division of the left posterior macromere (basal 

 cell) forms the mesoblast cell. (See Figs. 2-5.) 



