108 The Lungs, Heart, and Blood-vessels of the Slug. 



passed from the heart into arteries, from these into capillaries, 

 and from the latter into the veins, and thus returned to the 

 heart again. This view, as we shall see presently, was erro- 

 neous. It was at first held most persistently by Cuvief,* who, 

 although he had demonstrated the absence of capillaries in the 

 genus Aplysia (sea-hare), contended, nevertheless, that this 

 was but an exceptional instance, and that for the most part 

 head-bearing mollusks had a complete circulatory system. In 

 this century the most formidable controversy upon the ques- 

 tion has taken place between M. De Quatrefages and M. 

 Souleyet ; the former maintaining, correctly enough, that in 

 these beings the circulation is not perfect, and the latter assert- 

 ing with equal determination that it is perfect. f 



I have paid no inconsiderable attention to this subject, and 

 I believe I have succeeded in showing the course of the circu- 

 lation, and also the absence of capillary vessels.^ The blood 

 having been expelled from the heart (Fig. 1), passes through 

 the first great artery (aorta), and from it through its various 

 divisions and sub-divisions till it reaches the stomach, intes- 

 tines, head, liver, etc., etc., and at last arrives at the ends of 

 the arterial vessels ;§ and as these are quite open, the fluid 

 escapes from them. What then becomes of it ? It flows 

 into the great abdominal cavity — that sac formed by the in- 

 tegument in which are placed the important organs, which 

 constitute the great bulk of the animal. Here it bathes the 

 digestive tube and the great glands, and sooner or later is 

 admitted into the veins, and travels to the lungs. The general 

 veius, like those of the lungs, are channels grooved in the 

 skin, and are not distinct vessels, such as the arteries. They 

 ramify in the abdominal integument, springing on either side 

 from a large lateral branch which commences at the tail and 

 terminates in the lung, and by which all the blood that had 

 been thrown out from the arterial tubes is re-collected and 

 carried to tho respiratory surface. 



At this point difficulty has been invariably experienced in 

 ascertaining the precise direction in which the stream of blood 

 flowed — one side contending that of the entire current, a por- 

 tion passed to the so-called kidney, while the remainder flowed 

 through a special pulmonary vessel to the heart; tho other, 

 that the blood was poured at once into a sinus or lacuna. 

 Both these ideas J believe to be incorrect, tho more so as I 



* Rhgne Animal, " Mollusquos," p. 50.- 

 \ f Comp. Rend. xix. unci xx. 



J Quarterly Journal of Microscopical Science, January, 1803. 



§ Professor Milne Kd wards says, thai in most gastropods the aorfa ends in a 

 Binus (or cavity) containing the brain, gullet, and salivary glands, vide Frorieps 

 Neue Notizen, xxxiv. pp. 80, 2G0. This is not tlio case in the slug. 



