XXV111 INTRODUCTION. 



stout hollow tube, the free oviduct leading onwards to the genital 

 aperture (gen.ap\ the prostate or sperm-duct becoming a slender 

 tube, the vas deferens (vd) carrying the spermatic contents to the 

 male organ. We have now reached the vicinity of the head and 

 buccal mass. Before the free oviduct reaches the genital aperture 

 it gives off a more or less globose or pear-shaped sac, varying 

 much in size ; this is the spermatheca (sp). It receives, when the 

 animals are in coitu, the spermatophore, which issues from the male 

 organ of the other individual. The male organ (p) consists of a 

 muscular reversible sheath or tube extending a short distance from 

 the genital aperture backwards, and gives off, generally where it 

 doubles back, a strong muscle called the retractor penis muscle. 

 The next part contracts into a smaller length of tube, the epi- 

 phallus, to which the vas deferens joins after this latter has 

 formed a loop forward and close up to the genital aperture. At 

 the junction of the vas deferens and the epiphallus is usually a 

 caecum-like appendage, the kale-sac, or flagellum when more whip- 

 like. Within this the spermatophore is formed. Yet another 

 organ, not universally present, remains to complete this com- 

 plicated system, viz. the amatorial organ or dart-sac (am.or). It 

 is usually tough and muscular, cylindrical in form, with a 

 retractor muscle at the distal end ; within it is a pointed style, 

 which is protrusive at the genital orifice. This is analogous 

 to the calcareous dart of the Helicidae. The male organ and 

 adjacent parts is reversible and protruded externally during 

 copulation. 



The spermatophore (fig. ii, D) is a complex structure built up 

 or moulded within the walls of the flagellum during the period 

 of generative activity. It consists of two distinct parts one 

 long and gutter-like, generally spined ; the other a thin-walled 

 cylindrical sac in which the spermatozoa passing down the vas 

 deferens collect and are retained until transplanted into the 

 sperm-sac of another individual to fructify its ova. The spermato- 

 phore takes on many various forms in different genera and is as 

 yet quite unknown in the great majority of the Land-Mollusca ; 

 it would appear to be secreted rapidly and its remains broken up 

 are soon absorbed and disappear. 



The eggs when laid hatch out naturally by the warmth of the 

 atmosphere, but in some genera an ovoviviparous habit has come 

 about, and the eggs are retained in the oviduct (fig. ii, p. xxvi, B, & 

 fig. 2, p. 3) and go through the early stage of development there, 

 so that the young have well-formed shells and are able to crawl 

 about and feed themselves as soon as they are born. 



The shell makes its appearance is very early embryonic life, in 

 the form of a little rounded inverted cup or bowl, and as the 

 infolded viscera begin their spiral growth it is built up on the free 

 outer edge (the peristome). It is deposited by the edge of the 

 mantle in different layers, the external either horny or membran- 

 aceous ; in many cases it is very hard, glassy, and polished. In 

 ovoviviparous genera mentioned above, where the shell attains 



