INTBODUCTION. XXIU 



Above this are the very pronounced parallel peripodial grooves, 

 generally two, in a few eases three, broken into oblong spaces 

 (vide fig. 38, A, p. 75). The rest of the body is covered with 

 papillate tubercles or fine rugosities, separated by deep grooves 

 leading from the upper peripodial groove towards the dorsum of 

 the foot. The Zouitidas also have a conspicuous caudal mucous 

 gland variously formed (fig. 57, C, p. 150). The upper surface of 

 the foot behind may be keeled above, .but it is generally rounded. 

 Even in those genera of the Zonitidae, such as Girasia (fig. i, 

 A, B, p. xx), where the shell has been reduced to a mere membrane, 

 the foot never reaches the true slug-like stage found in Limax, 

 Avion, &.Q., iu which genera the viscera fill the whole foot to 

 its extreme posterior point, or rather the visceral hump is spi'ead 

 over the whole dorsal surface of the creeping-organ. The foot in 

 the Eastern forms of Zonitidae is solid in form, and their other 

 anatomical details present so great a similarity to the component 

 parts in the shell-bearing genera from which they have descended, 

 that they furnish closer hnks in a chain of evolution than is often 

 to be met with. 



The Musculab System. 



The greater mass of the body is a network of muscular tissue 

 most perfect and complicated. The larger muscles are very tough, 

 ribbon-like, smooth, and shiny. The largest is the shell-muscle 

 attaching the animal to the shell, and most of the stronger most 

 important muscles are given off" from it. In those species where 

 there is only a remnant of a shell and no columella the attach- 

 ments of the principal muscles lie around the thickened mantle- 

 edge, principally on the posterior margin ; these include the 

 buccal retractor and the retractors of the foot and eye-tentacles 

 (fig. i, B, C). Some 4 or 5 fine muscles lead from the sides of 

 the mouth and pass through the nervous collar in a posterior 

 direction. Thei'e is a localized series in the buccal mass known 

 as the depressor, protractor, and levator muscles. 



The Nervous System. 



The main mass of the nervous system is situated just behind 

 the buccal mass (fig. 2, h, p. 3) and mouth, forming a ring or 

 collar {c(j) (fig. 4, nrj, p. 4), through which the alimentary canal 

 passes posteriorly. It is made up of paired ganglia, more or less 

 united by commissural bands completing the collar. According to 

 their position above or below the esophagus ((«), they have been 

 termed the cerebral or supra- oesophageal ; the pedal with the 

 visceral or parieto-splanchnic, sub-oesophageal. Taking the genus 

 Girasia to exemplify the nervous system in the Zonitid® (fig. i, 

 C, D), these ganglia are all, as it were, coalesced together, and 

 the side connectives are not seen. The several nerves leading 

 from the upper or dorsal side of the mass define the cerebral 



