198 ZOOLOGY 
tide, they are stated always to return to it before the water 
has again receded. 
The visceral hump is covered by the conical shell. The 
body-wall at its edge is produced into a fold, the mantle. The 
ventral surface of the animal consists of the muscular oval 
Fic. 118. Diagram of a vertical median section of a Limpet, Patella vulgata. After 
Lankester—Zoological Articles reprinted from the Hncyclopedia Britannica. 
1. Mouth. 10. Heart in pericardium. 
2. Odontophore. 11. Nephridium. 
3. Radula. 12. Opening of larger nephridium. 
4, Radula sac. 13. Branchial efferent vessel (vein). 
5. Buccal cavity. 14. Branchial afferent vessel (artery). 
6. Laminated stomach. 15. Salivary gland. 
7. Intestine cut across. 16. Generative gland. 
8. Liver. 17. Edge of the mantle. 
9. Anus. 
foot, between which and the mantle a groove exists which 
lodges the gills. The foot is attached to the shell by a 
circular muscle which is incomplete anteriorly. 
A distinct head exists, and this carries a pair of tentacles 
with a pair of eyes which appear as black specks near the base 
of the tentacles. Above the head the groove between the foot 
and the mantle deepens into a large pallial cavity. Into this, not 
in the median line, but slightly to the left of it, the anus opens, 
and on each side of the anus lie the openings of the renal 
organs (Fig. 119). On the neck are also situated two small 
bodies representing the ctenidia, which are fully developed in 
the allied forms Haliotis and Fisswrella; in connection with 
these a patch of olfactory epithelium, the osphradium, has also 
been discovered. The function of these ctenidia, the original 
breathing organs, has been assumed by certain folds of the 
mantle forming the actual gills. 
