The heart in Patella consists of a single auricle (not two as in Haliotis and Fissurella) and a ventricle; the former receives the blood from the branchial vein, the latter distributes it through a large aorta which soon leads into irregular blood-lacunae.
The existence of two renal organs in Patella, and their relation to the pericardium (a portion of the coelom), is important. Each renal organ is a sac lined with glandular epithelium (ciliated cell, with concretions) communicating with the exterior by its papilla, and by a narrow passage with the pericardium. The connexion with the pericardium of the smaller of the two renal organs was demonstrated by Lankester in 1867, at a time when the fact that the renal organ of the Mollusca, as a rule, opens into the pericardium, and is therefore a typical nephridium, was not known. Subsequent investigations carried on under the direction of the same naturalist have shown that the larger as well as the smaller renal sac is in communication with the pericardium. The walls of the renal sacs are deeply plaited and thrown into ridges. Below the surface these walls are excavated with blood-vessels, so that the sac is practically a series of blood-vessels covered with renal epithelium, and forming a meshwork within a space communicating with the exterior. The larger renal sac (remarkably enough, that which is aborted in other Anisopleura) extends between the liver and the integument of the visceral dome very widely. It also bends round the liver as shown in fig. 12, and forms a large sac on half of the upper surface of the muscular mass of the foot. Here it lies close upon the genital body (ovary or testis), and in such intimate relationship with it that, when ripe, the gonad bursts into the renal sac, and its products are carried to the exterior by the papilla on the right side of the anus (Robin, Dall). This fact led Cuvier erroneously to the belief that a duct existed leading from the gonad to this papilla. The position of the gonad, best seen in the diagrammatic section (fig. 13), is, as in other Aspidobranchia, devoid of a special duct communicating with the exterior. This condition, probably an archaic one, distinguishes the Aspidobranchia from other Gastropoda.
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| Fig. 10.—Nervous system of Haliotis; the visceral loop is lightlyshaded; the buccal ganglia are omitted. (After Spengel.) | |
ce, Cerebral ganglion. pl.pe, The fused pleural and pedal ganglia. pe, The right pedal nerve. ce.pl, The cerebro-pleural connective. | ce.pe, The cerebro-pedal connective. s, s′, Right and left mantle nerves. ab, Abdominal ganglion or site of same. o, o, Right and left olfactory ganglia and osphardia receiving nerve from visceral loop. |
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| Fig. 13.—Diagram of a vertical antero-postero median sectionof a Limpet. Letters as in figs. 6, 7, with following additions.(Lankester.) | |
q, Intestine in transverse section. r, Lingual sac (radular sac). rd, Radula. s, Lamellated stomach. t, Salivary gland. u, Duct of same. v, Buccal cavity | w, Gonad. br.a, Branchial advehent vessel (artery). br.v, Branchial efferent vessel (vein). bv, Blood-vessel. odm, Muscles and cartilage of the odontophore. cor, Heart within the pericardium. |
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| Fig. 14.—Vertical section in a plane running right and left throughthe anterior part of the visceral hump of Patella to show the two renalorgans and their openings into the pericardium. (J.T. Cunningham.) | |
a, Large or external or right renal organ. ab, Narrow process of the same running below the intestine and leading by k into the pericardium. b, Small or median renal organ. c, Pericardium. d, Rectum. e, Liver. | f, Manyplies. g, Epithelium of the dorsal surface. h, Renal epithelium lining the renal sacs. i, Aperture connecting the small sac with the pericardium. k, Aperture connecting the large sac with the pericardium. |
The digestive tract of Patella offers some interesting features. The odontophore is powerfully developed; the radular sac is extraordinarily long, lying coiled in a space between the mass of the liver and the muscular foot. The radula has 160 rows of teeth with twelve teeth in each row. Two pairs of salivary ducts, each leading from a salivary gland, open into the buccal chamber. The oesophagus leads into a remarkable stomach, plaited like the manyplies of a sheep, and after this the intestine takes a very large number of turns embedded in the yellow liver, until at last it passes between the two renal sacs to the anal papilla. A curious ridge (spiral? valve) which secretes a slimy cord is found upon the inner wall of the intestine. The general structure of the Molluscan intestine has not been sufficiently investigated to render any comparison of this structure of Patella with that of other Mollusca possible. The eyes of the limpet deserve mention as examples of the most primitive kind of eye in the Molluscan series. They are found one on each cephalic tentacle, and are simply minute open pits or depressions of the epidermis, the epidermic cells lining them being pigmented and connected with nerves (compare fig. 14, art. [Cephalopoda]). 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.