108 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



these are more conspicuous in some species than others, being especially- 

 well developed in A. Poleii. The outer wall of the pulmonary sac 

 is very vascular, the blood-vessels being arranged in a very definite 

 order. Their arrangement is constant in all species of Acavus. 



A. large blood- sinus surrounds the rectum and is continuous with 

 the sinus in the collar. From this space the afferent pulmonary 

 vessels {of. v.) arise ; they branch extensively, and ultimately break 

 up into a capillary network. Alternating regularly with the afferent 

 vessels and their branches are the efferent pulmonary veins (ef.v.). 

 The two largest of these run direct to the auricle (au.). The smaller 

 of the two vessels collects the aerated blood from the left side of the 

 lung, whilst the larger one, which originates from the right anterior 

 corner of the external wall of the pulmonary sac, collects blood from 

 the middle and portion of the right side of the lung. A few smaller 

 efferent vessels, which collect blood from a portion of the right side of 

 the lung, instead of pouring their contents into the auricle, run to the 

 convex (anterior) border of the kidney, where they break up into 

 capillaries, and, entering the substance of the kidney, form a renal 

 plexus. The renal capillaries reunite to form a single efferent renal 

 vessel, which lies along the concave (posterior) border of the kidney 

 and opens into the auricle very near the apertures of the efferent 

 pulmonary veins. 



The pericardium (pe.) is situated in the left posterior corner of the 

 pulmonary chamber, and encloses a well- developed heart. On the 

 right of the pericardium there is a large crescent-shaped kidney (£.). 

 The ureter (u.) arises from the left side of the kidney close to the 

 pericardium ; it lies on the convex (anterior) border of the excretory 

 organ and runs along the extreme posterior end of the pulmonary sac, 

 abutting on the visceral mass ; it then curves round and follows the 

 straight portion of the rectum for a short distance, opening into 

 the respiratory chamber by an aperture, the urinary pore (u.p.), 

 situated immediately behind one of the posterior efferent pulmonary 

 vessels. The position of this excretory aperture is very constant 

 in all species of Acavus. Up to this point the ureter is a closed tube, 

 but it is continued beyond the renal pore, as an open groove {u.g.), the 

 floor of which is markedly plicated ; this groove runs parallel with 

 the rectum, and eventually terminates just in front of the pulmonary 

 orifice [p. a.). 



The arterial system exhibits no important modification. The 

 ventricle gives origin to a common aorta, which divides into posterior 

 and anterior branches. The former is distributed to the liver and 

 posterior portions of the alimentary canal. The anterior aorta (a.a.) 

 passes beneath the first coil of the intestine, where it gives off 

 a branch to the posterior portion of the hermaphrodite duct ; it is then 

 continued forwards, supplying the genitalia, alimentary canal, buccal 

 mass, etc., with branches. The reproductive organs, on account of 

 their excessive size, receive several arteries derived from the anterior 

 aorta.. 



The muscular system needs no special description. The retractor 

 muscles, which are large, converge, with the exception of the retractor 



