Fig. [278].—Transverse sections through the proboscis of Ph. charybdaeus. A, Anterior, through the principal ganglionic mass (G); B, posterior, at the level of the sieve-hairs (h). Coec, Intestinal caeca; Dil. M, dilator muscles; N, inner nerve-ganglion, with circular commissure; N′, outer nerve; or, chitinous lining of oral cavity; R M, Ret.M, retractor muscles. (After Dohrn.)
Fig. [279].—Transverse section through the basal joint of the third leg in Phoxichilus charybdaeus, ♀. Cut, Cuticle; Hyp, hypodermis; Int, intestinal caecum; N, nerve-cord; Ov, ovary; Sept, septum. (After Dohrn.)
The oesophagus is followed by a long gastric cavity, which sends forth caecal diverticula into the chelophores (when these are present), and four immensely long ones into the ambulatory legs. The caeca are attached to the walls of the limb cavities, especially at their extremities in the tarsi, by suspensory threads of connective tissue, and the whole gut, central and diverticular, is further supported by a horizontal septal membrane, running through body and legs, which separates the dorsal blood-vessel and sinus from the gut, the nervous system and the ventral sinus, giving support also to the reproductive glands. A short and simple rectum follows the gastric cavity.
In Phoxichilus, which lacks the three anterior appendages in the female and the two anterior in the male, two pairs of caeca run from the gut into the cavity of the proboscis (Fig. [278], B, coec.).[[404]]
Circulatory System.—The heart has been especially studied by Dohrn in Phoxichilus. It consists of a median vessel running from the level of the eyes to the abdomen, furnished with two pairs of lateral valvular openings, and sometimes, though not always, with an unpaired one at the posterior end. The walls are muscular, but with this peculiarity that the muscular walls do not extend around the heart dorsally, in which region its lumen is only covered by the hypodermis and cuticle of the back. The blood-spaces of the body are separated into dorsal and ventral halves by the septal membrane already referred to, which is perforated in the region of the lateral processes by slits placing the two cavities in communication; this septal membrane runs through the limbs to their tips, and far into the proboscis, where it is attached to the edge of the superior antimere. The blood is a colourless plasma with several kinds of corpuscles, of which the most remarkable are amoeboid, actively mobile, often coalescing into plasmodia. The course of the circulation is on the whole outwards in the inferior or ventral sinus, inwards towards the heart in the superior, save in the proboscis, where the systole of the heart drives the blood forwards in the dorsal channel. The beat is rapid, two or three times in a second, according to Loman, in Phoxichilidium. Especially in the species with small body and exaggerated legs, the movement of the circulatory fluid is actuated more by the movements of the limbs and the contractions of the intestinal caeca than by the direct impulse of the heart.
Nervous System.—The nerve-chain consists of a fused pair of supra-oesophageal ganglia, which innervate (at least in the adult) the chelophores, and of ventral ganglia, whence proceed the nerves to the other limbs. The ganglia of the second and third appendages are fused with one another, sometimes also with the ganglia of the first ambulatory legs; the ganglia of the three posterior pairs of legs are always independent (though the development of their longitudinal commissures varies with the body-form), and they are succeeded by one or two pairs of ganglia, much reduced in size, situated in the abdomen, of which the posterior one innervates the muscles of the abdomen and of the anal orifice. Each lateral nerve divides into two main branches, which supply the parts above and below the septal membrane. The nerve-supply of the proboscis is very complicated. Its upper antimere is supplied from the pre-oral, its two lateral antimeres from the first post-oral, ganglion, and each of these three nerves divides into two branches, of which the inner bears six to eight or more small ganglia, which annular commissures passing round the pharynx connect one to another. Of these ganglia and commissures the anterior are the largest, and with these the outer lateral nerve-branches of the proboscis merge. The immediate origin of the nerves to the chelophores is from the median nerve that springs from the under side of the supra-oesophageal ganglion to run forward into the proboscis, but it is noteworthy that the chelophores receive twigs also from the lateral nerves of the proboscis which arise from the post-oral ganglia.
Eyes.—Eyes are the only organs of special sense known in the Pycnogons. The deep-water Pycnogons, in general those inhabiting depths below four or five hundred fathoms, have in most cases imperfect organs, destitute of lens and of pigment, so imperfect in many cases as to be described as wanting. It is rare for the eyes to be lacking in shallow-water species, as they are, for instance, in Ascorhynchus minutus, Hoek, dredged by the Challenger in 38 fathoms, but, on the other hand, it is no small minority of deep-water species that possess them of normal character and size, even to depths of about 2000 fathoms.
In all cases where eyes are present, they are simple or “monomeniscous” eyes, four in number, and are situated in two pairs on an “oculiferous tubercle,” sometimes blunt and low, sometimes high and pointed, placed on the so-called cephalothorax, or first, compound, segment of the body. The anterior pair are frequently a little larger, sometimes, as in Phoxichilidium mollissimum, Hoek, very much larger, than the posterior. The minute structure of the eye has been investigated by Dohrn, Grenacher, Hoek, and Morgan. The following account is drawn in the first instance from Morgan’s descriptions.[[405]]