Internal Structure.—In Phalangium the mouth leads upwards into a membranous pharynx, wider than that of Spiders, but narrowing into an oesophagus which passes between the cerebral and thoracic ganglionic nerve-masses. It then turns backwards over the thoracic ganglion, being slightly dilated at that point. Immediately afterwards it dilates into a flask-like gastric sac which occupies almost the whole width of the abdomen, and proceeds straight to the anus. Viewed from above, the shape of this sac is entirely concealed by the large number of caeca (thirty) to which it gives rise dorsally and laterally. The two largest of these caeca extend, parallel to each other, over the whole of the abdominal portion of the gastric sac, and are flanked by four lateral pairs of smaller caeca, while there is a cluster of small caeca covering the anterior and narrower portion of the flask-like stomach.
The large hepatic mass so conspicuous on opening dorsally the abdomen of a Spider is here entirely absent, but its functions are believed to be performed by certain wrinkled, tubular, longitudinally parallel bodies, about seven in number, closely applied to the under surface of the flask.
The masticating portions of the maxillae of the pedipalpi and the first pair of legs are hollow distensible sacs, often seen in a swollen condition in specimens kept in spirits. They are furnished, on the inner surface, with a horny ridge.
Owing to the fixity of the coxae of the legs, their maxillary plates are incapable of much lateral motion, but are rubbed against each other vertically.
Beyond the fact that the heart is a dorsal tube lying along the anterior two-thirds of the alimentary canal, and divided by constrictions into three well-marked and equal portions, little is known of the blood-system of these animals. It is probably essentially like that of Spiders, but the presence of a pericardial sac has not yet been established, nor has the course of the blood-vessels been described in detail.
As in other Arachnids, the principal ganglionic nerve-masses closely embrace the oesophagus. Immediately anterior to it, forming a conical mass with its base on the oesophagus, is the cerebral ganglion, while just behind it is the transverse portion of the large thoracic nerve-centre. In Phalangium opilio, according to Tulk,[[342]] a median nerve is given off from the apex of the cerebral mass (the paired nature of which is apparent) and bifurcates to the two eyes. Two lateral nerves proceed to certain organs near the origin of the second pair of legs, which were thought by the old writers to be lateral eyes, but which are now known to be glands for the manufacture of the odorous fluid which these animals can exude.
Fig. [232].—Nervous and respiratory systems of a Phalangid. Nerves black, tracheae white. c.g, Cerebral ganglion; g′, g″, g‴, ganglia supplying viscera; m.n, median abdominal nerve; oe, passage for oesophagus; st, stigma; th.g, thoracic ganglion; tr, main trunk of tracheae.
The thoracic ganglion expands, on either side of the oesophagus, into a mass which extends nearly as far forward as the apex of the cerebral ganglion. These lateral masses give off nerves to the appendages. From the back of the transverse portion proceed three nerves. The median nerve passes above the generative organs, and soon branches into two nerves which presently swell out to form ganglia of considerable size, beyond which they soon join again and give off an anastomosing network of nerve-fibres. The lateral nerves immediately branch. The outer branch dilates into a ganglion which supplies the external part of the generative organ. The inner branch, which is longer, also forms a ganglion the nerves from which are chiefly distributed to the under surface of the alimentary canal.
The respiratory organs consist of two large tracheal tubes with numerous branches, having their external openings or “stigmata” near the base of the fourth pair of legs. The two main tubes are directed forwards, and are mainly concerned with supplying the largely developed muscles of the legs. The distribution of branches to the abdomen is comparatively feeble. The particular arrangement of tubes in P. opilio, according to Tulk, may be seen in the accompanying figure. There are a pair of coxal glands, of excretory function, opening in the neighbourhood of the coxae of the third pair of legs.