In the cellular membrane interposed be tween the two muscular layers of the fin ap paratus, four or five large nerves are seen to run a tortuous course, and to divide into in numerable Eschricht likewise ob served a considerable blood-vessel derived from the ventricle of the heart (not from the auricle), mounting up and dividing to supply each fin.
_Respiration and Circulation. — According to Cuvier's views the fins of the Pteropoda have been very generally regarded as performing likewise the functions of branchim. " Their surfaces, seen with the microscope, present a net-work of vessels, so regular, so close, and so delicate that it is impossible to doubt their office : their connection with the internal ves sels and the heart, moreover, confirms this idea." # Cuvier's opportunities of investi gating this point of their anatomy were, how ever, very limited ; a single specimen only, and that long kept in spirits of wine, having been at his disposal. Eschriches researches do not at all confirm this view of their nature; and it appears clear that Cuvier mistook the net work of muscular fibres represented in the preceding figure for vascular ramifications.
The vessel likewise called by Cuvier " the branchial vein," and which he regarded as re turning the blood from the branchim to the auricle of the heart, Eschricht assures us, does not communicate with the auricle, but is de rived from the apex of the ventricle so as to be evidently arterial, and not venous, in its nature.
With regard to the connexion which exists between the fin-apparatus and the body of the Clio, it would appear that its central muscular basis passes directly through the neck, and is only attached to the surrounding parts by nerves, vessels, skin, and cellular membrane.
Nervous system.—The nerves of the Clio are very easily traced, seeing that they are not only of considerable size, but are likewise con spicuous, on account of their pale red colour, at least while the specimens are tolerably fresh.
The cesophageal ring lies in the neck above the centre of the fin-apparatus, and lodged in its dorsal excavation. It is composed of eight large and two small ganglia. Each gang lion is surrounded by a transparent invest ment, and is very evidently composed partly of a reddish and partly of a white nervous substance. Of the eight larger ganglia of the circum-cesophageal ring the two anterior (fig. 113. 30, 1) are situated close together, upon the dorsal aspect of the cesophagus ; the two posterior ( fig. 113. 30, 4) are likewise close
together, but beneath the cesophageal tube. Of the four intermediate ganglia, two are situ ated close together on each side of the cesoph agus (fig. 113. 30, 2, 3), so that when viewed superficially, either from the dorsal or ventral aspect, they have the appearance of forming one elongated mass. By means of a nervous band which connects them, the eight ganglia form a double ring, seeing that the two lateral pairs of ganglia are, as weil as the inferior, brought into communication with each other through the intervention of a cross branch which runs beneath the cesophagus.
In addition to the eight ganglia above men tioned there are likewise two small nervous masses (fig. 113. 30, 5), situated one on each side of the anterior pair, with which they are connected hy short nervous branches.
All the nerves given off from these centres seem to proceed from the ganglia nearest to their place of destination. From the anterior pair are derived all the nerves supplying the parts of the head and the eyes. From the lateral pairs the nerves of the fins are prin cipally given off, while the posterior pair fur nishes nerves to all the hinder parts of the body.
Eyes.— The eyes in Clio are situated upon the dorsal aspect of the body, in the constric tion which constitutes the neck. In this situation the skin is drawn deeply around them, so that they seem to be lodged in spe cial depressions appropriated to receive them. Each eye (fig.113. 31) has somewhat the shape of a bent cylinder, the two ends of which are of a spherical form. The external spherical ex tremity of this eye, which is transparent, and constitutes the cornea, stands prominently above the level of the skin. By transmitted light it is not difficult to distinguish the con struction of the interior. The middle third of the cylinder is generally of a dark colour, whilst the anterior and posterior extremities of the cylinder are comparatively transparent ; but, probably, in the recent animal, the dark pig ment extends back as far as the hinder end — anteriorly, it is easy to perceive the existence of a transparent lens ; but from the small size of the organ, it is difficult to make out their structure more completely. In connexion with these eyes, delicate muscular fasciculi may be traced radiating in different directions, which would seem to have the office of turning the eye-ball towards any particular object.