301 



The life history, particularly the earlier stages, of the common 

 pond snail (Limnceus stagnalis) of Europe has been worked out with 

 much care by Prof. Ray Lankester, his observations confirming 

 those of Lereboullet, Pouchet and others so far as they extended. 



The eggs of Lirnmeus are deposited in June on the under side 

 of water-plants, in capsules enclosing one, rarely two eggs, and 

 surrounded by a mass of jelly. After seg- 

 mentation the Gastrula (Fig. 141, ra, mouth ; 

 ec, ectoderm ; en, endoderm) is formed, the 

 primitive digestive cavity or mouth result- 

 ing probably from an infolding of the ecto- 

 derm. Lankester believes that this orifice or 

 mouth is temporary, the mouth of the adult 

 being a later production. The primitive Gastrula 7r the Pond 

 mouth closes as the embryo enters on the ^ ™ Snail. 

 Veliger state, in the earliest stages of which the embryo is oval 

 and surrounded by a ciliated ring, much as in the larval Trochus 

 (Fig. 140, A). This stat. is called h } Lankester the "Trocho- 



, i ■ . - : . : 



hatches in about twenty days after life begins. 



Professor Lankester confirms the suggestions already made hy 

 Gegenbaur, Morse and Salensky regarding the resemblance of the 

 larval mollusks to young worms. He remarks also that both the 

 Trochosphere and Veliger forms are "well known and character- 

 istic of various groups of Worms and LYhiimderms, and the hitter 

 is seen in its full development in the adult Rotifera, and in the 

 laival Gasteropoda and Pteropoda. The identity of the velum of 

 larval Gasteropods, with the ciliated disks of Rot ifera, seems to ad- 

 mit of little doubt, and it would Ik; well to have one term, r. a., 



provided with a polar tuft of long cilia. 



"The cell, polyblast (morula), gastrula, trochosphere, and vel- 



molluscan pedigree; they belong to its prse-molluscan history. 

 The foot, shell-gland, and the odontophore are organs which are 



