TIIH ANATOMY OF MK(; ALATKACTUS — KKSTKVKN. 427 



axis of the organ. The cavity opens by a large aperture into tlie 

 ureter (icr.) ; this is a niuscuhir pocket, somewhat ovoid in shape, 

 situated in front of the nephridium in the depth of the mantle 

 cavity. The aperture of the ureter is large and easily seen 

 {aj).) ; it can prol)al)]y bo closeil b}' the muscles surrounding it. 



The nephridial (jlaiid (PI. xxix., fig. 3, nefli. yl.) is situated 

 in, and about tlie middle of the wall dividing the peri- 

 cardium and nephridium ; it is laigest at the posterior 

 end of the pericardium, and is enclosed by the above-mentioned 

 wall, and it here hangs into that cavity as a thick lamella. 

 As the anterior end of the pericardium is reached this 

 lamella becomes smaller till the gland lies flat in the integu- 

 ment of the wall. The colour of the gland is dirty pink, and it 

 is of a spongy texture. Its shape in transverse section and 

 position may be gleaned by a glance at PL xxxix., fig. 3. A 

 large number of pores (;:>r.) allow the blood to pass from the 

 cavity of the nephridium into the gland. An injection thi'ough 

 one of these pores was seen to diffuse through the gland and 

 collect in the vena cava. 



No reno-pericardial canal., properly so called, exists, but a con- 

 nection between the two organs is found in the two branches of 

 the vena-cephalica. For reasons stated later on it is regai^ded as 

 unlikely that any exchange of contents takes place by this means. 



4. The Vascular System. 

 (PL xxxix., figs. 1 and 3.) 



The per^ca?•c/^^«?^ (p.c.) is large, and is not nearly filled by the 

 contained organs. Its shape is somewhat that of an Anodonta, 

 the umbones representing the dorsal surface ; since it is of this 

 shape it cannot be said to bound the mantle cavity, although 

 situated immediately behind it on the left hand side of the 

 nephridium. The outer left hand wall differs only from the 

 posterior part of the mantle in being slightly thicker. The right 

 hand or inner wall is common to both pericardium and nephri- 

 dium, and has been described with this latter organ. No peri- 

 cardial gland was found. 



The heart (PL xxxix., fig. 4) is of the usual monotocardian type. 

 The auricle (awr.) is thin- walled and smaller than the ventricle (v.), 

 and they might be aptly compared, as to shape, to two pears 

 attached at their thick ends. The atrio-ventricular valve is com- 

 posed of several small flaps ; that which prevents the return of 

 the blood from the aortpe into the ventricle is of the single flap 

 type. At their inception the aortse are fused together, but their 

 cavities are separated by a thick partition. There is no main 

 aorta, and the ventricle opens directly into both. A glance at 



