to the edge of the free mantle-skirt, is the conical shell. When the shell is taken away (best effected by immersion in hot water) the surface of the visceral dome is found to be covered by a black-coloured epithelium, which may be removed, enabling the observer to note the position of some organs lying below the transparent integument (fig. 5). The muscular columns (c) attaching the foot to the shell form a ring incomplete in front, external to which is the free mantle-skirt. The limits of the large area formed by the flap over the head and neck (ecr) can be traced, and we note the anal papilla showing through and opening on the right shoulder, so to speak, of the animal into the large anterior region of the sub-pallial space. Close to this the small renal organ (i, mediad) and the larger renal organ (k, to the right and posteriorly) are seen, also the pericardium (l) and a coil of the intestine (int) embedded in the compact liver.
On cutting away the anterior part of the mantle-skirt so as to expose the sub-pallial chamber in the region of the neck, we find the right and left renal papillae (discovered by Lankester in 1867) on either side of the anal papilla (fig. 6), but no gills. If a similar examination be made of the allied genus Fissurella (fig. 17, d), we find right and left of the two renal apertures a right and left gill-plume or ctenidium, which here as in Haliotis and Pleurotomaria retain their original paired condition. In Patella no such plumes exist, but right and left of the neck are seen a pair of minute oblong yellow bodies (fig. 6, d), which were originally described by Lankester as orifices possibly connected with the evacuation of the generative products. On account of their position they were termed by him the “capito-pedal orifices,” being placed near the junction of head and foot. J.W. Spengel has, however, in a most ingenious way shown that these bodies are the representatives of the typical pair of ctenidia, here reduced to a mere rudiment. Near to each rudimentary ctenidium Spengel has discovered an olfactory patch or osphradium (consisting of modified epithelium) and an olfactory nerve-ganglion (fig. 8). It will be remembered that, according to Spengel, the osphradium of mollusca is definitely and intimately related to the gill-plume or ctenidium, being always placed near the base of that organ; further, Spengel has shown that the nerve-supply of this olfactory organ is always derived from the visceral loop. Accordingly, the nerve-supply affords a means of testing the conclusion that we have in Lankester’s capito-pedal bodies the rudimentary ctenidia. The accompanying diagrams (figs. 9, 10) of the nervous systems of Patella and of Haliotis, as determined by Spengel, show the identity in the origin of the nerves passing from the visceral loop to Spengel’s olfactory ganglion of the Limpet, and that of the nerves which pass from the visceral loop of Haliotis to the olfactory patch or osphradium, which lies in immediate relation on the right and on the left side to the right and left gill-plumes (ctenidia) respectively. The same diagrams serve to demonstrate the streptoneurous condition of the visceral loop in Aspidobranchia.
Fig. 7.—The same specimen viewed from the left front, so as to show the sub-anal tract (ff) of the larger nephridium, by which it communicates with the pericardium. o, Mouth; other letters as in fig. 6.
Thus, then, we find that the limpet possesses a symmetrically disposed pair of ctenidia in a rudimentary condition, and justifies its position among Aspidobranchia. At the same time it possesses a totally distinct series of functional gills, which are not derived from the modification of the typical molluscan ctenidium. These gills are in the form of delicate lamellae (fig. 4, f), which form a series extending completely round the inner face of the depending mantle-skirt. This circlet of gill-lamellae led Cuvier to class the limpets as Cyclobranchiata, and, by erroneous identification of them with the series of metamerically repeated ctenidia of Chiton, to associate the latter mollusc with the former. The gill-lamellae of Patella are processes of the mantle comparable with the plait-like folds often observed on the roof of the branchial chamber in other Gastropoda (e.g. Buccinum and Haliotis). They are termed pallial gills. The only other molluscs in which they are exactly represented are the curious Opisthobranchs Phyllidia and Pleurophyllidia (fig. 55). In these, as in Patella, the typical ctenidia are aborted, and the branchial function is assumed by close-set lamelliform processes arranged in a series beneath the mantle-skirt on either side of the foot. In fig. 4, d, the large branchial vein of Patella bringing blood from the gill-series to the heart is seen; where it crosses the series of lamellae there is a short interval devoid of lamellae.
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