MOLLUSCA 



155 



particles is effected by the lattice-work of the ctenidia or 

 gill-plates. 



The heart of Anodon consists of a median ventricle em- 

 bracing the rectum (fig. 143, A), and giving off an anterior 

 and a posterior artery, and of two auricles which open into 

 the ventricle by orifices protected by valves. 



The blood is colourless, and has colourless amoeboid 

 corpuscles floating in it. In two Lamellibranchs, Solen, 

 (Ceratifolen) legumen and Area Nox, the blood is crimson, 

 owing to the presence of corpuscles impregnated with 

 haemoglobin (Lankester, 31). In Anodon the blood is 

 driven by the ventricle through the arteries into vessel- 

 like spaces, which soon become irregular lacunae surround- 

 ing the viscera, but in parts e.g., the labial tentacles and 

 walls of the gut very fine vessels with endothelial cell- 

 lining are found. The blood makes its way by large 

 veins to a venous sinus which lies in the middle line be- 

 low the heart, having the paired renal organs (nephridia) 

 placed between it and that organ. Hence it passes 

 through the vessels of the glandular walls of the nephridia 

 right and left into the gill-lameLUe, whence it returns 

 through many openings into the widely-stretched auricles. 

 A great deal more pre- 

 cision has been given to 

 accounts of the structure 

 of arteries, veins, and 

 capillaries in Anodon 

 than the facts warrant. 

 The course of the blood- 

 stream can only be some- 

 what vaguely inferred ex- 

 cept in its largest out- 

 lines. Distinct arterial 

 and venous channels can- 

 not be distinguished in 

 the gill-lamellae, in spite 

 of what Langer (52) has 

 written on the subject, 

 though it is highly prob- 

 able that there is some 



kind of Circulation in the 



mlla Tn trip filampnt 

 glllS. in me mamentS 



of the gill of Mytilus the 

 . , .. j- -j j 



tubular cavity is divided 

 by a more or less complete fibrous septum into two 

 channels, presumably for an ascending and a descend- 

 ing blood-current. The ventricle and auricles of Anodon 

 lie in a pericardium which is clothed with a pave- 

 ment endothelium (d, fig. 143). Veins are said by Keber 

 and others to open anteriorly into it, but this appears to 

 be an error. It does not contain blood or communicate 

 directly with the blood-system ; this isolation of the peri- 

 cardium we have noted already in Gastropods and Cephalo- 

 pods. A good case for the examination of the question as 

 to whether blood enters the pericardium of Lamellibranchs, 

 or escapes from the foot, or by the renal organs when the 

 animal suddenly contracts, is furnished by the Solen legu- 

 men, which has red blood-corpuscles. According to ob- 

 servations made by Penrose (53) on an uninjured Solen 

 legumen, no red corpuscles are to be seen in the pericardial 

 space, although the heart is filled with them, and no such 

 corpuscles are ever discharged by the animal when it is 

 irritated. 



The pair of nephridia of Anodon, called in Lamelli- 

 branchs the organ of Bojanus, lie below the membranous 

 floor of the pericardium, and open into it by two well- 

 marked apertures (e and/ in fig. 143). Each nephridium, 

 after being bent upon itself as shown in fig. 143, C, D, 

 opens to the exterior by a pore placed at the point marked 

 x in fig. 124, (5), (6). It is no doubt possible, as in the 



142 ._ Vertiall ^^ Oaaagl 



donta, about the mid-region of the foot, w, 

 mantle-flap ; br, onter, bY, inner gill-plate 

 _e^ composed of two lamellae;/, foot ; r, 

 ventricle of the heart ; a, auricle ; p, p', 

 pericardial cavity ; i, intestine. 



Gastropoda and Cephalopoda, for water to enter from the 

 exterior by the nephridia into the pericardium, but that 

 it ever does so is as yet not proved. What is certain 

 from the set of the ciliary currents is that liquid generally 



ae 



FIG. 148. Diagrams showing the relations of pericardium and nephridia in a 

 Lamellibranch such as Anodon. A. Pericardium opened dorsally so ao to 

 expose the heart and the Boor of the pericardial chamber d. B. Heart 

 removed and floor of the pericardium cut away on the left side so as to open 

 the non-glandular sac of the nephridium, exposing the glandular sac 6, 

 which is also cut into so as to show the probe /. C. Ideal pericardium and 

 nephridium viewed laterally. D. Lateral view showing the actual relation 

 of the glandular and non-glandular sacs of the nephridium. The arrows 

 indicate the course of fluid from the pericardium outwards, o, ventricle of 

 the heart ; 6, auricle ; 66, cut remnant of the auricle ; c. dorsal wall of the 

 pericardium cut and reflected ; c, renp-pericardial orifice ; /, probe intro- 

 duced into the left reno-pericardial orifice ; g, non-glandular sac of the left 

 nephridium ; A, glandular sac of the left nephridium ; t, pore leading from 

 the glandular into the non-glandular sac of the left nephridium ; t, pore 

 leading from the non-glandular sac to the exterior ; ac, anterior, 06, posterior, 

 cut remnants of the intestine and ventricle, 



passes out of the pericardium by the nephridia. One half 

 of each nephridium is of a dark-green colour and glandular 

 (h in fig. 143). This opens into the reflected portion which 

 overlies it as shown in the diagram fig. 143, D, t ; the latter 

 has non-glandular walls, and opens by the pore k to the 

 exterior. The nephridia may be more ramified in other 

 Lamellibranchs than they are in Anodon. In some they 

 are difficult to discover. That of the common oyster 

 has recently (1882) been detected by Hoek (54). Each 

 nephridium in the oyster is a pyriform sac, which commu- 

 nicates by a narrow canal with the urino-genital groove 

 placed to the front of the great adductor muscle ; by a 

 second narrow canal it communicates with the pericardium. 

 From all parts of the pyriform sac narrow stalk-like tubes 

 are given off, ending in abundant widely-spread branching 

 glandular caeca, which form the essential renal secreting 

 apparatus. The genital duct opens by a pore into the 

 urino-genital groove of the oyster (the same arrangement 

 being repeated on each side of the body) close to but distinct 

 from the aperture of the nephridial canal Hence, except 

 for the formation of a urino-genital groove, the apertures 

 are placed as they are in Anodon. Previously to Hoek's 

 discovery a brown-coloured investment of the auricles of 

 the heart of the oyster had been supposed to represent 

 the nephridia in a rudimentary state. This investment, 

 which occurs also in Mytilus but not in Anodon, may pos- 

 sibly consist of secreting cells, and may be comparable to 

 the pericardial accessory glandular growths of Cephalopoda. 

 Nervous System and Sense-organs. In Anodon there are 

 three well-developed pairs of nerve-ganglia (fig. 1 44, B and 

 fig. 1 24, (6)). An anterior pair, lying one on each side of the 



