AOT) OVIDUCAL SYSTEM IN THE LAMELLIBEANCHIATE MOLLESKS. 
35 
communication with the blood-vessels or other tubes in the animal’s body. Now the 
TJnio 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 TJnio 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 dia- 
meter, 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 structure 
in the Pyrula. But in the TJnio just spoken of as so fully injected, as well as in several 
others s imil arly 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 invo- 
lution or glandular depression in the foot of the TJnio as homologous with the foot- 
gland of the terrestrial Glasteropods ; and the communication which has been held to 
exist between this LameUibranchiate organ and its vascular system, we should not 
believe to be more direct than that which subsists between the muciparous foot-gland 
of the lAmax and its venous system f. 
It is not quite beside the purpose, to remark that the foot of one of the Unionidae, 
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 
ha\ing 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 
animal’s 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 existence 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 following 
words : — “ The existence in these animals of a double system of lacunae having this inter- 
pretation is attended with many difficulties. For then it must be admitted that one of 
these systems contains only water and the other blood, and it is difficult to understand 
how two kinds of waU-less canals can traverse the body without passing into each other.” 
It is, however, demonstrable that in the Unionidae, at all events, an all but perfectly 
closed system of blood-vessels exists. We have again and again, with various injecting 
fluids, found that they wiU pass from the aorta through a capillary system into a 
* Loc. cit. p. 238. Vox Hesslixg, however, does not mention the occurrence of calcareous concretions 
impacted in this gland’s duct. This we have observed. 
t SiEBOLD, Anatomy of Invertebrata, p. 255, note 6, American edition. 
F 2 
t Ibid. p. 213. 
