302 ANIMAL BIOLOGY. [Part II. 



in length; the clear transparent ends taper off to fine points. 

 In the middle is a white opaque portion, in which are the sper- 

 matozoa. At the time of congress the two snails exchange 

 spermatophores, which pass into the spermatheca, where they 

 seem to break up, giving exit to the spermatozoa, by which 

 the ova are fertilised, as they pass down the oviduct. The 

 muciparous glands, which give rise to a milky secretion, perhaps 

 supply them with cases. 



The darts are extruded at such times. They may sometimes 

 be found sticking in the skin of the snail ; while in dissecting 

 the snail their remains may sometimes be found in or near the 

 spermatheca or the genital duct. 



The ova are laid in groups, generally in June or July, and 

 in about 20 days hatch into young snails. 



Development of Pond Snail. The development of the pond 

 snail (Limnceus) is more typical than that of Helix, and its ova 

 and embryos are readily obtained, and studied without difficulty. 

 If pond snails be kept in a glass aquarium tank, there will, in 

 summer, be found adhering to the glass, or to water-weed, small 

 oval patches, or elongated strips of jelly-like substance, divided 

 into a number of compartments, within each of which is an 

 ovum or an embryo. 



Segmentation is holoblastic and fairly regular. First two 

 and then four segmentation spheres are formed. Then from the 

 upper surface of these four spheres there separate off smaller 

 spheres, which, by a continuation of this process and by sub- 

 division amongst themselves, become comparatively numerous. 

 In Fig. 88, A., there are four large spheres, macromeres (mac.), and 

 several smaller spheres, micromeres (mic.). Subsequently both 

 macromeres and micromeres increase in number, a segmentation 

 cavity is formed, and invagination takes place in such a way that 

 the macromeres are pushed in (Fig. 88, B.) and become hypoblast 

 (%.), the micromeres giving rise to epiblast (ep.). 



The blastopore (bl.) so formed would seem to elongate before 

 it finally closes, the position of the future mouth and anus being 

 at the anterior and posterior end of the slit. Both mouth 



