INTRODUCTION . XXVII 



near where the dorsal lobes meet. The pulmonary chamber or 

 cavity lies beneath the shell on the left side just behind the upper 

 margin of the peristome and continues back for some distance, its 

 general shape and size presenting variation in different genera. 

 The upper surface is an extremely transparent thin wall, showing, 

 in most species, a system of small veins uniting in a main 

 pulmonary vein (pv) and forming a respiratory surface by which 

 they breathe air directly. On the posterior left side of the 

 pulmonary chamber is the pericardium (fig. ii, A, jp). 



HEART AND CIRCULATORY SYSTEM. 



The pericardium (fig. ii, A, p) is a sac lying on the left anterior 

 side of the body in dextral shells, adjacent to the branchial cavity 

 and close against the renal organ (&), all these lying on the dorsal 

 surface of the body. Within it is the heart, composed of a single 

 auricle (ali) and a single ventricle (vh) : the first receives the blood 

 from the respiratory organs, which passes by a short constricted 

 valvular passage to the second. A short duct, the aortic trunk, 

 follows: it branches into two separate veins, the anterior and 

 posterior aorta, conveying the generally colourless blood to the 

 various organs of the body. Thence it collects in the venous 

 sinuses in the foot and viscera and the circumference of the body 

 before entering the respiratory (pv} and renal organs (fc). In 

 the former of these oxidation takes place and the blood returns 

 to the heart again by the pulmonary vein (pv). In the renal 

 organ waste products in the blood are taken up, which are thrown 

 out by way of a narrow passage (ko) lying parallel to the rectum 

 and so on to the anus (a). 



REPRODUCTIVE ORGANS. 



The generative orifice is situated close behind the right eye- 

 tentacle (fig. i, A, p. xx). The animal is hermaphrodits or moncecious, 

 the male and female systems being brought together in one indi- 

 vidual, and the animals unite for mutual fecundation. The source 

 of the system lies far back in the visceral sac (fig. i, A), where 

 buried in the posterior lobe of the liver is the ovo-testis (kg) 

 (a mass of cells), on the internal surface of which both ova and 

 spermatozoa are produced : the former are rounded cells, the 

 latter long and hair-like, with variously-shaped heads massed 

 together. From the ovo-testis extends a long duct called the 

 hermaphrodite duct (hd), usually very much convoluted, and 

 down which the ova and spermatozoa pass. The duct enters the 

 albumen-gland (alg), an elongate mass, and just before it does so 

 there is a small enlargement or sharp bending termed the seminal 

 vesicle. The ova here undergo a certain change and are separated 

 from the spermatozoa the one to pass down the oviduct, the 

 other down the prostate. 



The oviduct (ov) is of a whitish colour, with large convoluted 

 folds. These two coalesced ducts run side by side for a con- 

 siderable distance and then separate, the ovo-testis becoming a 



