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.
|Fig. 11.—Nervous system of Fissurella. (From Gegenbaur, after Jhering.)
pl, Pallial nerve.
p, Pedal nerve.
A, Abdominal ganglia in the streptoneurous visceral commissure, with supra- and sub-intestine ganglion on each side.
B, Buccal ganglia.
C, C, Cerebral ganglia.
es, Cerebral commissure.
o, Otocysts attached to the cerebro-pedal connectives.
|Fig. 12.—Diagram of the two renal organs (nephridia), to show their relation to the rectum and to the pericardium. (Lankester.)
f, Papilla of the larger nephridium.
g, Anal papilla with rectum leading from it.
h, Papilla of the smaller nephridium, which is only represented by dotted outlines.
l, Pericardium indicated by a dotted outline—at its right side are seen the two reno-pericardial pores.
ff, The sub-anal tract of the large nephridium given off near its papilla and seen through the unshaded smaller nephridium.
ks.a, Anterior superior lobe of the large nephridium.
ks.l, Left lobe of same.
ks.p, Posterior lobe of same.
ks.i, Inferior sub-visceral lobe of same.
|Fig. 13.—Diagram of a vertical antero-postero median section of a Limpet. Letters as in figs. 6, 7, with following additions. (Lankester.)
q, Intestine in transverse section.
r, Lingual sac (radular sac).
s, Lamellated stomach.
t, Salivary gland.
u, Duct of same.
v, Buccal cavity
br.a, Branchial advehent vessel (artery).
br.v, Branchial efferent vessel (vein).
odm, Muscles and cartilage of the odontophore.
cor, Heart within the pericardium.
|Fig. 14.—Vertical section in a plane running right and left through the anterior part of the visceral hump of Patella to show the two renal organs 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.
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).