474 THE ANIMAL KINGDOM. 



handed or sinistral twist to be normal in a given genus or species. The 

 successive coils of the spiral are closely applied to one another as a rule, 

 but the lower coils may become loose and straggling, e. g. Siliquaria, 

 Vermetus ; and in Magilus, when the animal becomes inclosed by the corals 

 among which it lives, the shell may lose its spiral growth altogether. The 

 adult of the Non-Palliate Opisthobranchia is devoid of any shell at all. The 

 Veliger, however, possesses a small nautiloid shell, which is discarded during 

 growth, and the same is true of the shell-less Azygobranchia Natantia. The 

 first formed shell aborts in some cases, and a second shell, unlike the first, 

 is then formed, e. g. Marsenia among Reptant Azygobranchia. The texture 

 of the shell varies. It is often brightly coloured, and the coloration is 

 derived from the glands at the edge of the mantle. Its anterior edge is 

 sometimes produced into a spout-like notch, along and beyond which a 

 grooved process of the mantle is capable of protrusion. This process is the 

 siphon, and is characteristic of the carnivorous sub-order Siphonochlamyda 

 of Azygobranchia Reptantia, e. g. the Whelk. 



The ectoderm, or epidermis, consists of a columnar epithelium pro- 

 vided with a delicate cuticula, and which, where it is exposed, is often 

 ciliated to a greater or less extent. It contains unicellular mucous glands, 

 pigment glands, and glands the secretion of which is loaded with Calcium 

 carbonate. The two latter kinds of glands are especially numerous on the 

 edge of the mantle, where they secrete the colouring matter of the shell, its 

 outer epicuticula, and the greater part of its calcareous substance. But the 

 cells of the surface of the mantle and visceral dome also have the power of 

 repairing injuries to the parts of the shell already formed. The mucous 

 glands are sometimes large, and then lie in the sub-epidermic connective 

 tissue. A series of transverse lamellae is developed in many Streptoneura 

 to the right of the ctenidium on the roof of the branchial cavity. They 

 constitute the hypobranchial gland, and secrete much mucus. In the 

 genera Purpura and Murex the secretion, at first colourless, changes in 

 sunlight to a purple or violet, used as a dye by the ancients, and known 

 as * Tyrian purple.' Specialised glands are found in connection with the 

 foot, and may be distinguished into a suprapedal gland, which opens 

 between the head and foot, e.g. Helix, Succinea among Pulmonata, 

 Vermetus ) Cycles toma among Azygobranchia Reptantia ; and a pedal gland, 

 which opens on the surface of the foot itself, as in certain of the latter 

 group, e. g. Conus, Nassa, Bithynia. Other special glands are the grape- 

 like poison glands of Aplysia and Dolabella, which open near the genital 

 aperture close to the osphradium, and the small glands on the inner surface 

 of the mantle-edge which secrete a purple liquid in Aplysia Camelus, a 

 colourless liquid in other species of the genus and its allies. In Phyllirhoe 

 certain unicellular cutaneous glands exude a fatty phosphorescent secretion. 

 The muscles are composed of long non-striated muscle-cells. The foot is 



