Page:EB1911 - Volume 11.djvu/528

Rh The limpet breeds upon the southern English coast in the early part of April, but its development has not been followed. It has simply been traced as far as the formation of a diblastula which acquires a ciliated band, and becomes a nearly spherical trochosphere. It is probable that the limpet takes several years to attain full growth, and during that period it frequents the same spot, which becomes gradually sunk below the surrounding surface, especially if the rock be carbonate of lime. At low tide the limpet (being a strictly intertidal organism) is exposed to the air, and (according to trustworthy observers) quits its attachment and walks away in search of food (minute encrusting algae), and then once more returns to the identical spot, not an inch in diameter, which belongs, as it were, to it. Several million limpets—twelve million in Berwickshire alone—are annually used on the east coast of Britain as bait.

Sub-order 1. Docoglossa.—Nervous system without dialyneury. Eyes are open invaginations without crystalline lens. Two osphradia present but no hypobranchial glands nor operculum. Teeth of radula beam-like, and at most three marginal teeth on each side. Heart has only a single auricle, neither heart nor pericardium traversed by rectum. Shell conical without spire.


 * Fam. 1.—Acmaeidae. A single bipectinate ctenidium on left side. Acmaea, without pallial branchiae, British. Scurria, with pallial branchiae in a circle beneath the mantle.


 * Fam. 2.—Tryblidiidae. Muscle scar divided into numerous impressions. Tryblidium, Silurian.


 * Fam. 3.—Patellidae. No ctenidia but pallial branchiae in a circle between mantle and foot. Patella, pallial branchiae forming a complete circle, no epipodial tentacles, British. Ancistromesus, radula with median central tooth. Nacella, epipodial tentacles present. Helcion, circlet of branchiae interrupted anteriorly, British.


 * Fam. 4.—Lepetidae. Neither ctenidia nor pallial branchiae. Lepeta, without eyes. Pilidium. Propilidium.


 * Fam. 5.—Bathysciadidae. Hermaphrodite; head with appendage on right side; radula without central tooth. Bathysciadium, abyssal.

Sub-order 2. .—Aspidobranchia with a palliovisceral anastomosis (dialyneurous); eye-vesicle closed, with crystalline lens; ctenidia, osphradia and hypobranchial glands paired or single. Radula with very numerous marginal teeth arranged like the rays of a fan. Heart with two auricles; ventricle traversed by the rectum, except in the Helicinidae. An epipodial ridge on each side of the foot and cephalic expansions between the tentacles often present.


 * Fam. 1.—Pleurotomariidae. Shell spiral; mantle and shell with an anterior fissure; two ctenidia; a horny operculum. Pleurotomaria, epipodium without tentacles. Genus includes several hundred extinct species ranging from the Silurian to the Tertiary. Five living species from the Antilles, Japan and the Moluccas. Moluccan species is 19 cm. in height.


 * Fam. 2.—Bellerophontidae. 300 species, all fossil, from Cambrian to Trias.


 * Fam. 3.—Euomphalidae. Also extinct, from Cambrian to Cretaceous.


 * Fam. 4.—Haliotidae. Spire of shell much reduced; two bipectinate ctenidia, the right being the smaller; no operculum. Haliotis.


 * Fam. 5.—Velainiellidae, an extinct family from the Eocene.


 * Fam. 6.—Fissurellidae. Shell conical; slit or hole in anterior part of mantle; two symmetrical ctenidia; no operculum. Emarginula, mantle and shell with a slit, British. Scutum, mantle split anteriorly and reflected over shell, which has no slit. Puncturella, mantle and shell with a foramen in front of the apex, British. Fissurella, mantle and shell perforated at apex, British.


 * Fam. 7.—Cocculinidae. Shell conical, symmetrical, without slit or perforation. Cocculina, abyssal.


 * Fam. 8.—Trochidae. Shell spirally coiled; a single ctenidium; eyes perforated; a horny operculum; lobes between the tentacles. Trochus, shell umbilicated, spire pointed and prominent, British. Monodonta, no jaws, spire not prominent, no umbilicus, columella toothed. Gibbula, with jaws, three pairs of epipodial cirri without pigment spots at their bases, British. Margarita, five to seven pairs of epipodial cirri with a pigment spot at base of each.


 * Fam. 9.—Stomatellidae. Spire of shell much reduced; a single ctenidium. Stomatella, foot truncated posteriorly, an operculum present, no epipodial tentacles. Gena, foot elongated posteriorly, no operculum.


 * Fam. 10.—Delphinulidae. Shell spirally coiled; operculum horny; intertentacular lobes absent. Delphinula.


 * Fam. 11.—Liotiidae, shell globular, margin of aperture thickened. Liotia.


 * Fam. 12.—Cyclostrematidae. Shell flattened, umbilicated; foot anteriorly truncated with angles produced into lobes. Cyclostrema. Teinostoma.


 * Fam. 13.—Trochonematidae. All extinct, Cambrian to Cretaceous.


 * Fam. 14.—Turbinidae. Shell spirally coiled; epipodial tentacles present; operculum thick and calcareous. Turbo. Astralium. Molleria. Cyclonema.


 * Fam. 15.—Phasianellidae. Shell not nacreous, without umbilicus, with prominent spire and polished surface. Phasianella.


 * Fam. 16.—Umboniidae. Shell flattened, not umbilicated, generally smooth; operculum horny. Umbonium. Isanda.


 * Fam. 17.—Neritopsidae. Shell semi-globular, with short spire; operculum calcareous, not spiral. Neritopsis. Naticopsis, extinct.


 * Fam. 18.—Macluritidae. Extinct, Cambrian and Silurian.


 * Fam. 19.—Neritidae. Shell with very low spire, without umbilicus, internal partitions frequently absorbed; a single ctenidium; a cephalic penis present. Nerita, marine. Neritina, freshwater, British. Septaria, shell boat-shaped.


 * Fam. 20.—Titiscaniidae. Without shell and operculum, but with pallial cavity and ctenidium. Titiscania, Pacific.


 * Fam. 21.—Helicinidae. No ctenidium, but a pulmonary cavity; heart with a single auricle, not traversed by the rectum. Helicina. Eutrochatella. Stoastoma. Bourceria.


 * Fam. 22.—Hydrocenidae. No ctenidium, but a pulmonary cavity; operculum with an apophysis. Hydrocena, Dalmatia.


 * Fam. 23.—Proserpinidae. No operculum. Proserpina, Central America.

Order 2. .—In this order there is no longer any trace of bilateral symmetry in the circulatory, respiratory and excretory organs, the topographically right half of the pallial complex having completely disappeared, except the right kidney, which is