THE PULSE OF MOLLUSCA. 221 



cate with lacunae or sinuses in the intervisceral spaces, or with 

 those in the skin." But the veins communicate with, and 

 originate from, a system of small sinuses forming a network 

 which pervades the whole of the skin, "being abundant on 

 the sides under the bases of the papillae, and on the foot, 

 and we suppose communicates freely with the system of in- 

 tervisceral lacunae pointed out by Milne -Edwards." " The 

 general course of the blood will be necessarily then from 

 the ventricle along the arteries to the viscera and to the skin ; 

 in the first case it passes from the arteries in a way we do 

 not understand, into the lacunae among the viscera and be- 

 tween them and the skin, and thence into the network of 

 sinuses in the skin itself, in the latter case into the tegumen- 

 tary sinuses ; in them, and in the papillae, into which it is 

 freely admitted, it is more or less perfectly aerated, and 

 thence flows into the veins which pass from the skin to the 

 auricle, and which are called by M. Milne-Edwards branchio- 

 cardiac vessels. From what we have observed, however, on 

 attentively examining the connections of the ovarium, we 

 are inclined to think that the whole of the blood does not 

 circulate in the way above described, for we are pretty sure 

 we have recognised small veins passing away from the sides 

 of the ovarium and entering the skin, and we mentioned above 

 that we had, though indistinctly, made out a pair of veins 

 running from the same organ to the posterior trunk vein, 

 that empties itself into the auricle. If these observations be 

 correct, then a small portion of blood is returned to the 

 heart in a way that forms an exception to the general rules, 

 and the existence of veins distinct from the branchio-cardiac 

 is established."* 



The heart has been seen pulsating in several mollusks 

 whose bodies possess the requisite degree of transparency 

 that exposes the internal viscera to our gaze. The pulsa- 

 tions are usually slow, and often at unequal intervals, but 

 this irregularity is dependent on the state of the animal in 

 relation to action or repose, or it may be the effect of weak- 

 ness or of pain, for generally the individual must be placed 

 in an unnatural position, or removed from its proper element, 

 before the observations can be made ; and an attention to this 

 circumstance may explain the fact of a retrograde motion of 

 the circulating fluid, which has been observed by some expe- 

 rimentalists. Lister found that the heart, when removed 

 from the Snail, ceased to pulsate after some hours, but being 

 moistened with saliva, the pulsations were renewed ; and this 



* Ann. and Mag. N. Hist. sec. 2, i. 97- 103. 



