and Oviducal System in the Lamellihranchiate Mollush, 453 



blood-vessels or other tubes in the anmiaPs body. Now the 

 Unio margaritifera stands in the same relation with reference to 

 these foot-pores to the Anodonta cygnea as the Pyrula carica and 

 P. canaliculata do, according to Agassiz_, to the Mactra; that is 

 to say, the foot of the Unio presents us with a gigantic pore, in 

 the shape of a glandular depression of as much as an inch in 

 length and two lines in diameter, whilst that of the Anodon is 

 pierced but by microscopic inlets. Von Hessling*, by whom 

 this organ has been very accurately described, believes that 

 injections can be made to pass, without rupture of any limitary 

 membrane, from its cavity into the blood-vessels ; and Agassiz 

 holds a similar view with reference to the nearly similar struc- 

 ture in the Pyrula. But in the Unio just spoken of as so fully 

 injected, as well as in several others similarly treated, though 

 the sides and walls of this glandular depression were very richly 

 injected, none of the injection could by pressure be made to 

 issue out into the water in which the animal was lying. We 

 should be inclined to consider this involution or glandular de- 

 pression in the foot of the Unio as homologous with the foot- 

 gland of the terrestrial Gasteropods ; and the communication 

 which has been held to exist between this Lamellihranchiate 

 organ and its vascular system we should not believe to be more 

 direct than that which subsists between the muciparous foot- 

 gland of the Limax and its venous system f. 



It is not quite beside the purpose, to remark that the foot of 

 one of the Unionidse, when thoroughly distended, has a smooth 

 bright appearance, so uniformly spread over the whole surface of 

 its semigelatinous mass as to suggest the idea of the depressions 

 having become everted and thus contributed to increase the size 

 of the infiltrated organism. Though this appearance may not 

 justify such an interpretation, yet it does seem quite inconsistent 

 with the existence of patent pores communicating with the 

 animaVs blood-vessels. 



We have repeatedly observed that, if a freshwater mussel die 

 with its muscular foot in a state of contraction, no distention of 

 the foot takes place, either by leaving the animal to soak in 

 water till putrefaction sets in, or by artificial injection. 



We will now proceed to state our reasons for holding the exist- 

 ence of a water-vascular system distinct from the blood-vessels 

 of the Lamellibranchiata. Siebold J states one of the objections 

 urged against the existence of this system of vessels in the fol- 



* hoc. cit. p. 238. Von Hessling, however, does not mention the occur- 

 rence of calcareous concretions impacted in this gland's duct. This we 

 have observed. 



t Siebold, Anatomy of Invertebrata, p. 255, note 6, American edition. 



X Ibid. p. 213. 



