54 LILLIE. [Vol. X. 



function be not that of excretion in the strict sense of the 

 word, we would have the anomaly of an active larva without 

 any provision for the excretion of waste products. The waste 

 products in this case take the form of an insoluble substance 

 as an adaptation to development within the strong vitelline 

 membrane where soluble waste products could not but act 

 injuriously. The thread-like form of the waste substance is 

 the mechanical result of the form of the secreting gland. If 

 the thread really assists the larva in attaining its host, this is 

 a secondary and subordinate function. 



Of the ectodermal structures of this stage it remains only to 

 consider the rudiment of the larval mantle. This includes all 

 the cells from the blastopore to the opening of the thread- 

 gland. Its lateral extension is well shown in Fig. 79, all of 

 the cells below the shell being included in it. It will be seen 

 that the oral plate lies about in the middle of the area in 

 question. The cells are cylindrical with the nuclei about the 

 middle of their height. They resemble each other throughout 

 the whole extent of the area, excepting in one narrow strip in 

 the median line extending from the opening of the thread-gland 

 to the oral plate (Fig. 80 s.c!). The cells in question were 

 noticed by Flemming and called by him suture-cells (" Naht- 

 zellen"). They are very narrow long cells and mark the line 

 of division of the right and left halves of the mantle. 



About this time (Figs. 79 and 80) appear the bristles which 

 later become such a characteristic feature of the glochidium. 

 There are but three pairs of these organs at this time (Figs. 

 79 and 80). Each is a little bunch (three to five in number) 

 of stiff hairs born by a special cell. The position of these six 

 cells is : one on each side of the thread-gland ; one on each 

 side of the oral plate, and one on each side of the anterior end 

 of the ventral plate. A fourth pair is soon formed beneath the 

 first pair mentioned. 



2. Mesoblastic Organs. 



We left the mesoblast in the stage in which eight cells are 

 formed : four of the primary {M) and four of the larval meso- 

 blast (F) (Figs, 64, 65, 72, etc). The cleavage of the parent 



