52 LILLIE. [Vol. X. 



the stage of Fig. 82) numerous mesoderm cells apply them- 

 selves to the gland and completely encase it. The further 

 growth and secretion seems to be supported by these cells. 

 The extrusion of the thread takes place some time before the 

 rupture of the vitelline membrane. 



One of the earliest and most natural ideas in regard to this 

 organ was, that it was the homologue of the byssus-gland of 

 other lamellibranchs. Rabl seems to have adopted this idea ; 

 he at any rate uses the term byssus-gland. Carriere (No. 10) 

 was the first to show that this view is untenable, both from 

 the position of the organ, and also from the fact that an actual 

 byssus-gland is formed in the parasitic larva. He came to the 

 only tenable conclusion, viz., that it is an organ siii geiieris. 

 As such it is still regarded, and I have found no characteristic 

 which would cause it to rank with other organs elsewhere. 

 But the morphology of the region which it occupies is very 

 imperfectly understood ; it is this latter question which I wish 

 to clear up. 



Ziegler (No. 6^) suggested that the three bladder-like cells 

 described by Rabl just in front of the shell-gland were a rudi- 

 mentary head-vesicle. In the third volume of their text-book, 

 Korschelt and Heider advance the same idea. This suggestion 

 contains but part of the truth ; they are, in fact, but part of 

 a rudimentary head-vesicle. The head-vesicle in molluscan 

 larvae is the region in front of the velum, which passes just 

 in front of the shell-gland and of the mouth on the dorsal and 

 ventral sides, respectively. The homologous region is thus 

 easily defined in Unio. It is represented by the larger part 

 of the region in front of o.p. (PI. V, Figs. 69, 73, 79), and of 

 the shell-gland or shell. It is thus seen to be quite an exten- 

 sive area. The thread-gland opens at its dorsal limit in the 

 middle line. It thus lies very nearly in the course which the 

 velum would take if present, and I was therefore at first inclined 

 to interpret its protoblast, with the similar cells surrounding it 

 (Fig. 71), as cells of a rudimentary velum. However, com- 

 parison with Ziegler's figure (No. 6t, Fig. 6) convinced me 

 that the cells in question were really cells of a rudimentary 

 head-vesicle. If Ziegler's Fig. 6 be compared with my Fig. 69, 



