LACUNAE. 537 



those lacunse are very dilatable, and afford space for a considerable 

 accumulation of fluid ; in the dorsal region, on the contrary, they are 

 small, and more densely congregated. It is this structure which con- 

 stitutes the aquiferous system of Delle Chiaje ; but it has no commu- 

 nication with the exterior of the body. The membrane, which imper- 

 fectly lines the abdominal cavity, separates this structure from the 

 visceral chamber, but does not cut off the communication that exists 

 between them ; on the contrary, the peritoneal tunic is itself of a spongy 

 texture, and is perforated with numerous apertures, whereby a free 

 passage is established between the subcutaneous lacunse and the interior 

 of the abdomen. In this way it happens that, when a coloured fluid is 

 injected into the visceral cavity, the whole lacunary system becomes 

 filled; and on throwing injections, even of coarse materials, into the 

 muscular interstices of the foot or mantle, they are seen at once to diffuse 

 themselves through the abdominal cavity. 



(1421.) From the above and similar facts, Milne-Edwards has satis- 

 factorily established the following important conclusions : 



1st. That no complete vascular system exists in any of the Mollusca. 



2nd. That throughout a greater or less extent of the circulatory circle 

 veins are entirely wanting, their functions being performed through the 

 medium of lacunse, or by the great cavities of the body. 



3rd. That frequently the veins are wanting altogether, and that in 

 such cases the blood distributed through the body by the arterial system 

 can only return to the respiratory surface by the intervention of the 

 interstitial lacunae above described. 



(1422.) Professor Huxley, in a letter addressed to Professor Milne- 

 Edwards*, relative to the circulation of the blood, expresses himself 

 very decidedly upon this important point in the anatomy of the Mol- 

 lusca. In Firola, one of the Heteropod division, he observes that, 

 owing to the perfect transparency of the body of this mollusk whilst 

 alive, nothing is more easy than to observe the circulation of the blood 

 throughout its entire course. In this creature no veins whatever are 

 observable. The globules of the blood may be seen to issue in crowds 

 from the open termination of the arteries of the foot, through the 

 substance of which they immediately become diffused, and may likewise 

 be observed to pass from the mass of the mouth, in which the aorta 

 terminates, directly into the peri-intestinal cavity, in which they may 

 be seen to return gently, frequently stopping in their course towards the 

 heart. Occasionally some of them may be traced directly into the 

 auricle, passing through the interspaces between the network of muscu- 

 lar fibres composing its walls f, in the meshes of which they may some- 



* Ann. des Sci. Nat. 1850. 



f In Firola, Professor Huxley assures us, the walls of the auricle of the heart are 

 composed of a kind of lacework made up of striated and ramified muscular fibres, 

 between which large open spaces are observable. 



