57 



norvegica, the vunbonal part is so large as to occupy a 

 considei*able part of the lambonal region of the shell cav- 

 ity; in X. f imbriata it is small, and in T. navalis , appa- 

 rently rudimentary. In his studies of the pericardial 

 glands in Lamellibranchs , Grobben sought in Teredo for the 

 gland riescribed by Deshayes in the umbonal region, think- 

 ing it might represent a part of the pericardial gland of 

 other forms which possess this organ. He failed to find 

 it and supposed it to be absent. Hov/ever, though he ap- 

 parently had none of the forms with which I have worked, 

 I think it was doubtless present in his species. 



In the larva th.is peculiar structure is present on 



either side in front of the cerebral ganglion, though in a 



form 

 much simplified (g.D. fig. 2). It is vesicular and fil- 

 led with spherical cells of apparently nucous nature. A 

 duct leads to the exterior, opening at the side of the 

 mouth, on the ventral side of the velum. 



The structure of subsequent stages oi' the gland will 

 be best linderstood by first describing that part in the 

 gill. A glance at fig. 38 will show that the modified por- 

 tion contains elements of two very different types of 

 structure. Their distribution and relations are best 

 shown in fig. 37, which represents a lamina from the gill of 



