692 MOLLUSCA [LAMELLIBRANCHIA. 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 (C eratisolen) leyumen and Area Nose, 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-lamellae, 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 i j i T Fid. 142. Vertical section through an Ano- kmd 01 Circulation in the donta, about the mid-region of the foot, m, trills In the filaments mantle-flap ; br, outer, bY, inner gill-plate i , . . each com posed of two lamellae ;/, foot; v, of the gill of MytlTUB the ventricle of the heart ; , auricle ; p, p , tubular cavity is divided P ericardil11 <*** : *> intestine - 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- pocls. 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 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 3 I Hi k r: FIG. 143. Diagrams showing the relations of pericardium and nephridia in a Lamellibranch such as Anodon. A. Pericardium opened dorsally so as to expose the heart and the floor 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 ft, 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, a, ventricle of the heart ; b, auricle ; bb, cut remnant of the auricle ; c, dorsal wall of the pericardium cut and reflected ; e, reno-pericardial orifice ; /, probe intro duced into the left reno-pericardial orifice ; g, non-glandular sac of the left nephridium ; h, glandular sac of the left nephridium ; i, pore leading from the glandular into the non-glandular sac of the left nephridium ; k, pore leading from the non-glandular sac to the exterior ; ac, anterior, ab, 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 (k in fig. 143). This opens into the reflected portion which overlies it as shown in the diagram fig. 143, D, i ; 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. 144, E and
fig. 124, (6)). An anterior pair, lying one on each side of the