JOURNAL OF MICROSCOPICAL SCIENCE. 



EXPLANATION OF PLATE XVII, 



Illustrating Mr. Ray Lankester's Memoir on the Develop- 

 ment of the Pond-snail. 



Fig. 1. — An embryo between the Trochosphere and Veliger phases — 

 somewhat compressed ; longest diameter about ^^ inch. m. Mouth. 

 V. Velum. /. Foot, g e. Gastrula-endoderm, now assuming a bilobed 

 character — the sac enclosed by it having therefore a double cavity, a right 

 and a left, g. Point of closure of the gastrula-mouth. p i. Pedicle of in- 

 vagination — the future rectum, s p. The shell-patch — a thickened area of 

 the body waU, on the surface of which the shell first forms, and by invagi- 

 natipn of which the shell-gland is produced. 



Fig. 2. — Young Veliger, surface view, showing — m. Mouth, v. Velum. 

 f. Foot. Longest diameter about y^ inch. 



Fig. 3. — A similar specimen from the oral aspect. 



Fig. 4. — A similar specimen somewhat compressed and seen in incom- 

 plete optical section, m. Mouth. /. Foot, p i. Pedicle of invagination. 

 s h. Shell overlying the depression of the shell-gland, which is now visible. 



Fig. 5. — Surface view of a more advanced Veliger, longest diameter 

 about ^ of an inch. /. The foot, showing bilobation. v. Velum now 

 forming a heart-shaped area, with m, the mouth, at its base. t. The eye- 

 tentacles. 



Fig. 6. — Surface view of a more advanced embryo, in a position fre- 

 quently assumed at this period of development. Length of the specimen, 

 about ^ inch. fp. A doubtful structure, lying between the two lobes 

 of the foot, possibly a foot-pore. Other letters as above. 



Fig, 7. — A much more advanced embryo, about ^^ of an inch long, 

 f. Foot. fp. Foot-pore. v. Velum now assuming the character of " sub- 

 tentacular lobes." t. Tentacles. I. Lung- chamber, m f. Mantle-flap, 

 or free border of the mantle, sh. Shell, 



Fig, 8, — An embryo, a little older than that of fig, 5, Length about 

 ^ of an inch, compressed, treated with osmic acid, and seen in partial 

 optical section, m. Mouth, ph. Pharynx, ods. Odontophore-sac. /. Foot. 

 p i. Pedicle of invagination (rectum) with csecal termination, mf. Mantle- 



