438 MOLLUSCA . SUB-KINGDOM vi 



breathing pore. The sides of this pore, in operculated snails, are often pro- 

 duced outwards, so as to form a closed or cleft tube, corresponding with which 

 there is frequently a canal-like process of the shell. 



Gastropods are remarkable for their extreme differentiation of the repro- 

 ductive organs. The sexes are distinct in the Prosobranchia and Heteropoda, 

 but united in the Opisthobranchiata, Pteropoda, and Pulmonata. The ovarian 

 and seminal ducts of hermaphrodites sometimes open into a common cloaca, 

 or they may terminate in separate openings. 



The shell, as has already been remarked, is secreted by the mantle, and is 

 limited in form and size by the configuration of the intestinal sac. It is 

 composed of a chitinous substance (concliiolin) infiltrated with lime carbonate, 

 or exceptionally with sulphate of lime in small quantities. Shell characters 

 are of great importance in distinguishing genera and species, but their value 

 in classifying larger groups is comparatively slight, owing to the fact that, 

 very similar shells are often developed among forms which differ widely in 

 their general organisation. Two forms of shell-habit occur, the symmetrical 

 and the spiral. The first are flat-, conical, or saucer-shaped, and characterise 

 only a few groups (Cyclobranchia, Aspidobranchia, Pulmonata). Transition 

 forms between the symmetrical and spiral are to be observed in conical shells 

 with slightly inrolled beaks. Exceptional forms of the spiral shell are seen 

 in FermetuSj which is irregularly coiled, and in Planorbis, Bellerophon, and 

 Atlanta, coiled in one plane (discoidal). Usually the shell forms a screw-like 

 spiral, and rests upon the back of the creature in such a way that the apex is 

 directed upward and backward, the aperture forward and downward. Holding 

 the shell upright so that the apex is above, and the aperture below, facing 

 the observer, it is said to be right-handed or dextral when the opening is on 

 the right side, and left-handed or sinistral when on the left side. By far the 

 larger number of Gastropods are dextral ; but a few (Clausilia, Physa, Spirialis) 

 are normally sinistral. Eight-handed individuals of normally left-handed 

 genera, as well as pathologic individuals of normally right-handed forms, are 

 occasionally met with. 



In drawing and describing Gastropod shells, the apex is ordinarily directed 

 upward, so that the right- ,or left-handedness may be seen at a glance. It is 

 also customary to employ the terms above and below in the same sense as 

 posterior and anterior. The height or length of the shell is measured by a 

 line drawn from the apex to the lower margin of the aperture. 



The shell is to be considered as a more or less rapidly widening cone, 

 which is wound either around an axial pillar, called the columetta, or about a 

 central tubular cavity. Each coil of the tube is termed a whorl, and all the 

 whorls except the last one form together the spire. The last or body whorl 

 is often very much larger than the preceding ; its lower, sometimes flattened 

 surface is called the base. As a rule, the whorls are in contact with each 

 other, each in succession either partly or entirely covering the preceding ; but 

 in rare cases they form a loose spiral, in which the whorls are separated from 

 one another. The spire is said to be convolute when the later whorls 

 entirely conceal the earlier ones, as in Cypraea. The line between two con- 

 tiguous whorls is known as the suture. According to the manner of inrolling, 

 various shell contours are produced, requiring numerous descriptive names, 

 such as conical, auriform, turbinate, fusiform, cylindrical, spherical, oval, 

 pyramidal, etc. 



