This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1167). The muscular system of Pentelasmis is partly appropriated to the movements of the shell and partly to the general motions of the body. The shell is closed by a single transverse fasciculus of muscular fibres, whereof a section is seen at e, fig. 232, placed immediately beneath that fissure in the mantle through which the arms are protruded; it passes directly across from one valve to the other, and approximates them by its contraction.
Fig. 233. Pentelasmis vitrea, exhibiting the nervous system: a a, the pedicle; b b, the body; c, the mouth; d d, glandular masses;ff, visceral nerves; o o, nervous collar surrounding the oesophagus; h, i, k, I, m, series of double ganglia supplying the articulated cirri. (After Cuvier).
* Cuvier, loc. cit.
(1168). A large muscle, whose origin is seen in fig. 232, f, arises from the interior of the mantle, and, as its fibres diverge, spreads over the entire mass of the viscera; this will evidently draw the body forward and cause the protrusion of the tentacula; while various muscular slips derived from it scarcely need further description, being destined to move the numerous arms, with their jointed cirri and the fleshy tubular prolongation (fig. 235, 7c) already noticed.
(1169). The food devoured by the Cirrhopoda would seem to consist of various minute animals, such as small Mollusks and microscopic Crustacea, caught in the water around them by a mechanism at once simple and elegant. Any one who watches the movements of a living Cirrhopod will perceive that its arms, with their appended cirri, are in perpetual movement, being alternately thrown out and retracted with great rapidity, and that, when fully expanded, the plumose and flexible stems form an exquisitely beautiful apparatus, admirably adapted to entangle any nutritious molecules, or minute living creatures, that may happen to be present in the circumscribed space over which this singular casting-net is thrown, and drag them down into the vicinity of the mouth, where, being seized by the jaws, they are crushed and prepared for digestion. No sense but that of touch is required for the success of this singular mode of fishing; and the delicacy with which the tentacula perceive the slightest contact of a foreign body shows that they are eminently sensible to tactile impressions.
As regards the digestive organs, we have already described the prominent mouth (fig. 235, b), with its horny palpiferous lip and three pairs of lateral jaws. The oesophagus (c) is short, and firm in its texture; it receives the excretory ducts of two salivary glands of considerable size (fig. 233, d d), and soon terminates in a capacious stomachal receptacle, the walls of which are deeply sacculated and surrounded by a mass of glandular caeca (fig. 235, d) that represent the liver, and pour their secretion through numerous wide apertures into the cavity of the stomach itself. The intestine (e, f) is a simple tube, and runs along the dorsal aspect of the animal, wide at its commencement, but gradually tapering towards its anal extremity; it terminates at the root of the tubular prolongation (k) by a narrow orifice, into which a small bristle (g) has been inserted.
(1170). Little is satisfactorily known relative to the arrangement of the blood-vessels and the course of the circulation in these animals. Poli imagined that he had discovered a contractile dorsal vessel, intimating that he had perceived its pulsations in the vicinity of the anal extremity of the body; and although his observations upon this subject have not been confirmed by subsequent investigations, analogy would lead us to anticipate the existence of the heart in the position indicated by the indefatigable Neapolitan zootomist. The lateral appendages (fig. 232, d d d) are most probably proper branchial organs, but, perhaps, not exclusively the instruments of respiration, since the numerous cirri no doubt cooperate in exposing the blood to the action of the surrounding medium, a function to which they are well adapted by their structure and incessant movements; especially as each cirrus is seen under the microscope to be traversed throughout its whole length by two large vascular trunks, one apparently arterial, and the other of a venous character.
(1171). Judging from the peculiar conditions under which the Cir-rhopods exist, it would only be natural to suppose that to creatures so circumstanced, the possession of the organs of the higher senses would be a useless incumbrance, seeing that they are apparently quite incapable of holding communication with the external world; nevertheless, from the recent discoveries of Professor Leidy* and of Mr. Darwin1, they are found to be by no means destitute in this respect. In Lepas fascicularis, Mr. Darwin detected two nervous filaments, derived immediately from the front of the two supra-cesophageal ganglia, which were found to terminate in two small, perfectly distinct, oval masses, which are not united by any transverse commissure. From the opposite ends of these two ganglia, smaller nerves are derived, which, bending inwards at right angles, communicate with the ocular apparatus, which, although apparently consisting of a single mass, is, in reality, composed of two eyes united together; or, in other words, although in outline the eye appears single, two lenses can be distinctly seen at the end, as well as two pigment-capsules, which are deep and cup-shaped, and of a dark reddish-purple hue.
This double eye, in all the genera examined, is seated deep within the body: it is attached by fibrous tissue to the radiating muscles of the lowest part of the oesophagus, and lies actually on the upper part of the stomach; consequently a ray of light to reach the eye has to pass through the exterior membrane and underlying corium and to penetrate deeply into the body. In living sessile Cirri-peds, Mr. Darwin observes, vision seems to be confined to the perception of the shadow of an object passing between them and the light; they instantly perceive a hand passed quickly at the distance of several feet between a candle and the vessel in which they may be placed.