No. I.] THE EMBRYOLOGY OF THE UNI ONI DAE. 6 1 



tufts of Stiff, sensory hairs, arranged in a very characteristic 

 manner. Three pairs lie just beneath, and within the power- 

 ful hooks ; these three form the angles of a right-angled 

 triangle, the base of which is parallel to the transverse plane of 

 the larva, the apex being directed anteriorly. The fourth pair 

 lie on either side of the opening of the thread-gland. These 

 are, undoubtedly, the four tufts described for the young larva ; 

 but how different in their relative positions ! As in the young 

 larva, each tuft is borne by a single cell, the base of which is elon- 

 gated in a peculiar manner, to be described more fully later on. 



Near the posterior angle of the valves are two ectodermal 

 pits — the lateral pits. Between them, beneath the ectoderm, 

 is the entodermal sac. Behind this lie the lateral wings of 

 mesoderm cells. The ventral plate occupies the whole of the 

 posterior median region as far forward as the oral plate, which 

 has now assumed the form of a stomadeal invagination. Just in 

 front of the oral plate is the opening of the thread-gland, from 

 which the long, much-tangled larval thread has been extruded. 



The transformation of the young larva into the glochidium is 

 attended by a series of shiftings and displacements of cells and 

 groups of cells, which make this part of the development 

 extremely difficult to follow. Flemming is the only author 

 who has described these processes in detail; and indeed his 

 description leaves little to desire in some ways. But Flemming 

 was so uncertain as to the morphological meaning of the larval 

 parts that he gave them all special names: " Wimperschild " = 

 ventral plate ; " Vorderwulst " = entodermic sac ; " Mittel- 

 schild " = oral plate. It is due to this failure on the part of 

 Flemming to recognize the homologies of these various parts 

 that the apparent neglect of this part of his -work is due. 



The early appearance of the four paired tufts of hairs is of 

 great assistance in following these changes. The arrangement 

 of the sensory hairs on their first appearance has already been 

 described. Their final arrangement may be seen in Fig. 93. 

 To recapitulate : At first they are arranged in a row on each 

 side, as follows : one tuft to the side of the thread-gland aper- 

 ture, and a second tuft a little below this; the third lies just 

 above the oral plate, and the fourth to the sides of the ento- 



