MORPHOLOGY OF MELIBE 553 



in the archaic gasteropod Neomenia, the pericardial and 

 perigonadial coelomic remnants are continuous, and form one 

 cavity. There is strong reason to beheve that in ancestral 

 molluscs the haemocoel was more completely tubular and truly 

 vasiform than it is in living molluscs. In the later molluscs 

 the walls of the vessels have swollen out in many regions 

 (especially in the veins) and have obliterated the coelom, 

 which have shrunk to the small dimensions of the pericardium 

 and perigonadium. There are, however, many molluscs with 

 complete capillaries, arteries, and veins, in certain regions of 

 the body. 



(2) The Heart and Arteries. 



While the intestine in M. leonina, by its diagonal course 

 through the visceral cavity, disturbs the apparent bilateral 

 symmetry, the heart is situated in the median line, just anterior 

 to the anus (PI. 31, fig. 33), and to the left of the intestine. The 

 heart consists of two chambers, a dorsal and a ventral (PI. 34, 

 fig. 55, All, Vent). The dorsal chamber is the smaller of the 

 two ; it is partitioned off into small spaces through which 

 the lilood is returned by the efferent branchial veins {Au). 

 These chambers may be called auricular chambers ; they are 

 perforated (Av), and the partitions (Ar) which may serve as 

 valves may also close the perforations. The partitions or 

 valves with their apertures are so arranged that the openings 

 do not coincide with each other, and are therefore easily 

 closed. The ventricle or the larger of the two cardiac chambers 

 has a regular A^alve at its lower and constricted portion (PL 34, 

 fig. 59, Valve). The heart, therefore, may be completely 

 closed upon the contraction by the valves of the two chambers. 

 The heart is enclosed within the pericardium which also 

 encloses the efferent branchial veins (PL 27, figs. 9, Au, V; 

 PL 34, fig. 54, Au, Per, Vent). The aorta passes from the 

 floor of the ventricle (figs. 9, 54, Ao) to the ventral region 

 of the visceral cavity where it divides into two anterior and 

 posterior trunks (fig. 54, Aa). Just A^entral to the ventricular 

 valve is an enlargement of the wall, the structure of which 



