694 MOLLUSCA [LAMELLIBRANCHIA. left, continuous with the wall of the tubular branches of the gland (fig. 124, (5), (6), y). In no Lamellibranch is there a divergence from this structure, excepting that in some (Ostrea) the contiguous nephridial and the genital aperture are sunk in a urino-genital groove, which in other cases (Spondylus?) may partially close up so as to con stitute a single pore for the nephridial and genital ducts. No accessory genital glands are present. The development of Anodon is remarkable for the curious larval form known as Glochidium (fig. 147). The Glochidium Fig. 148. Embryos of Pisidium pitsillum (after Lankester). A. Only four embryonic cells are present, still enclosed in the egg envelope. B. The cells have multiplied and commenced to invaginate, forming a blastopore or orifice of invagination, U. quits the gill-pouch of its parent and swims by alternate opening and shutting of the valves of its shell, as do adult Pecten and Lima, trailing at the same time a long Fig. 150. Fio. 140. Embryo of Fisidium pusillum in the diblastula stage, surface view (after Lankester). The embryo has increased in size by accumulation of liquid between the outer and the invaginated cells. The blastopore has closed. Fio. 150. B. Same embryo as fig. 149, in optical median section, showing the invaginated cells hy which form the arch-enteron, and the mesoblastic cells me which are budded off from the surface of the mass hy, and apply them selves to the inner surface of the deric or epiblastic cell-layer ep. C. The same embryo focused so as to show the mesoblastic cells which immediately underlie the outer cell-layer. byssus thread. By this it is brought into contact with the fin of a fish, such as Perch, Stickleback, or others, and effects a hold thereon by means of the toothed edge of its shells. Here it becomes encysted, and is nourished by the exuda tions of the fish. A distinct development of its internal organs has been traced by the late Professor Balfour, but no one has followed it to the moment at which it drops from the fish s fin and assumes the form of shell characteristic of the parent. Other Lamellibranchs exhibit either a trocho- sphere larva which becomes a Veliger, differing only from the Gastropod s and Pteropod s Veliger in having bilateral shell-calcifications instead of a single central one ; or, like Anodon, they may develop within the gill-plates of the mother, though without presenting such a specialized larva as the Glochidium. An example of the former is seen in the E FIG. 151. Further stages in the development of Pisidium pusillum (after Lankester). A. Optical section of an embryo in which the foot has begun to develop. B. The same embryo focused to its surface plane to show the mouth o. C. Later embryo, showing the shell-gland sh. D. Lateral view of the same embryo. E. Later stage, with rudiments of the mantle-flap, lateral view. F. Still later stage, with shell-valves and branchial filaments, ep, epiblast ; me, mesoblast ; al, met-enteron ; rp, rectal peduncle or pedicle of invagination connecting the met-enteron with the cicatrix of the blastopore ; o, mouth; ph, pharynx; sh, shell-gland; mn, mantle-flap; br, branchial filaments ; y, granular cells of doubtful significance ; v, vesicular structure of unknown significance. development of the European Oyster, to the figure of which, and its explanation the reader is specially referred (fig. 6). An example of the latter is seen in a common little fresh- water bivalve, the Pisidium pusillum, which has been studied by Lankester (12). The successive stages of the development of this Lamellibranch are illustrated in the woodcuts figs. 148 to 153 inclusive. These should be compared with the figures of Gastropod development (figs. 3, 4, 5, 7, and 72***). Fig. 148 shows the cleavage of the egg-cell into four (A), and at a later stage the tucking
in of some of the cells to form an invaginated series (B).