222 transactions liverpool biological society. 



Lacunae and Venous Sinuses. 



The lacunae are irregular spaces in the tissues into which 

 the arteries empty themselves, there being neither capillary 

 system nor system of veins. The lacunae are best shown in 

 those parts of the body, such as the foot, the parapodia, the 

 tentacles or the mantle, where the tissues are of a spongy 

 nature. Here there are interlacing bands of fibrous and 

 muscular tissue united by connective tissue. Between the 

 bands are numerous irregular spaces, the lacunae. The only 

 compact tissue in these regions is the surface epithelium. The 

 lacunae are not bounded by any epithelium, so that, as in the 

 case of the walls of the heart, the muscles and connective 

 tissue fibres are apparently bathed by the blood. By means of 

 this lacunar system a rapid flow of blood into the tissues can be 

 effected, causing sudden turgidity and expansion. This was 

 thought to be due, by earlier writers, to the imbibition of 

 water. 



In the visceral hump the lacunae are much smaller owing 

 to the compactness of the organs in this region. It is note- 

 worthy, however, that the connective tissue holding the parts 

 of the visceral hump in place is fenestrated so that the blood 

 can pass through. 



The course of the blood from the lacunae back to the 

 heart can be traced by injection or by sections of small speci- 

 mens. If, for example, a few drops of Prussian blue solution 

 be carefully injected by means of a hypodermic syringe into 

 any portion of the foot or lateral body wall, the fluid readily 

 passes through the lacunae and emerges into the general body 

 cavity in the form of little spurts from gaps between the 

 longitudinal muscle bands of the foot and sides of the body. 

 (Fig. 21, fed. lac.) It can similarly be shown that the blood 

 from the mantle, the body wall generally, the tentacles, the foot 

 and parapodia is discharged by irregularly-placed apertures 



