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HANDBOOK OF INVERTEBRATE ZOOLOGY. 



and the mantle there is a little ridge (si), the inner 

 siphon fold. 



In a day or so more the mantle (Fig. 194, m) is sharply 

 defined, and begins to overhang the mantle-cavity, under 

 which the rudimentary gills (g) have appeared as little 

 papillae covered with cilia. They are very similar in struc- 

 ture to the embryonic gill-tentacles of a lamellibranch. In 

 the centre of the mantle there is a small pit, the shell- 

 gland (s), in which the shell or pen 

 soon appears, as a little flat, trans- 

 e$ parent, circular plate (Fig. 195, s). 



FiG. 194. Foreshortened dorsal view of 

 the posterior surface of the embryo shown in 

 Fig. 193. (Drawn from nature by W. K. 

 Brooks. ) 



y, s, and ?. as in the preceding figure. 

 es. Eye-stalk. g. Gills. er. Otocysts. 

 si. Inner siphon-folds, a. Arms. 



Fio. iw. 



The eye-stalks (Figs. 194 and 195, es) now project con- 

 siderably from the sides of the dorsal end of the body. 

 The eye invaginations are well developed, and their open- 

 ings have begun to close up, to form the pupils, in which 

 the lenses are soon developed. In Fig. 195 the left eye is 

 shown in surface view and the right in profile. The 

 lateral siphon-folds (si f ) are well developed, and the two 

 internal siphon-folds (Fig. 194, si) have appeared on the 

 posterior surface of the body. Opposite the outer ends 

 of the inner siphon-folds the ears (er) are now present as 

 two spherical pits, with wide mouths. On the anterior 

 surface, the mouth (Fig. 195, mo) is now visible, and a 

 short oesophagus runs inwards and upwards from it towards 

 the dorsal surface, but soon ends blindly. On each side 



