MOLLUSCA 



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). TheGlochidium 



157 



Fig. 148.— Embryos of Fisidivm pusillwm (after Lankester). A. Only four 

 embryonic cells are present, still enclosed in the egg envelope. B. The cells 

 have mnltipUed and commenced to invaginate, forming a blastopore or orifice 

 of invagmation, W. 



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. 149.— 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. 



Fig. 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 efiects 



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, hke 

 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 



FiQ. 151.— Further stages in the development of PisicHwm 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 0. C. Later esibryo, showing the shell-gland sh. D. Lateral view of 

 the same embryo. E. Later stage, with rudiments of the mantle-flap, lateral 

 view. P. 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 ; 

 0, 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 Fisidium 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). 



