No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 53 



a very striking agreement will be found in this respect. Just 

 in front of the cells of the shell-gland in both cases are large 

 rounded cells with large nuclei (text, Figs. 7 and 8 h.v). In 

 Cyclas these cells enter into the formation of the head-vesicle. 

 In Unio, the head-vesicle not being developed, we can only 

 regard these cells as a part rudiment of that once important 

 structure. 



The cell which forms the thread-gland is thus one of the 

 cells of the head-vesicle. No such use is made of these cells 

 elsewhere and so the organ must retain its rank as morpho- 

 logically siii gejieris. But is it so physiologically } According 

 to the generally received idea that the function of the thread 

 is merely to assist the larva in attaching itself to its host, it is. 

 It seems to me, however, that this cannot be its sole function. 

 It certainly cannot have been the primitive function ; for if it 

 be of such assistance to the larva, it can only be in virtue 

 of the length and strength of the thread secreted, and such 

 an extensive structure could hardly come into existence fully 

 formed. Its primitive function, both ontogenetically and phylo- 

 genetically, was probably excretion. Let us see what evidence 

 there is in the actual development. I have already called atten- 

 tion to the rows of granules which radiate from the nucleus 

 toward the lumen of the gland in early stages (PI. V, Fig. 74). 

 I take these to be an indication of active secretion on the part 

 of the gland.i But after the terminal nucleus has disappeared, 

 and this kind of excretion has ceased, matter still continues to 

 be excreted through the gland, only now in the form of a solid 

 substance, the thread. It is not necessary to assume that 

 the function of excretion is lost simply because the products 

 of excretion are utilized. Instances of utilization of waste 

 products (so called) on the part of the animal producing them 

 are by no means rare. This is moreover the only active larval 

 organ which communicates freely with the exterior. If its 



1 It may, perhaps, be worth while to call attention to somewhat analogous 

 function of cells of the prae-trochal region in Nereis. Wilson has discovered that 

 two cells lying not far from the apical plate but behind it, move into the cavity 

 of the head vesicle, where they acquire a lumen. He interprets them as head- 

 kidneys ; with a certain reservation be it said. These cells are paired, the thread- 

 gland of Unio is, however, a median structure. 



