(10) ARACHNIDA - ORDER III: PHALANGIDEA - INTERNAL STRUCTURE
INTERNAL STRUCTURE. - Muscular System. - Every one who has seen the mode in which the Phalangides run would naturally suppose that the muscles required to keep the body raised and balanced between their long slender legs in rapid course must be exceedingly strong; and this is found to be the case. The muscles are of large size, consisting of numerous strong "fasciculi," which arise from the interior of the basal joints (coxae) of the legs, and, almost entirely filling up their hollows, pass on into the next joint (trochanter), and so on to the end of the second part of the tibiae; thence two long delicate tendons, continuing forwards, traverse the whole length of the tarsal joints, running along their under surface. The muscles by which the falces and palpi are worked consist of an elevator and a, somewhat larger, depressor for each. Other muscles and muscular fibres connected with other parts and organs are minutely described by A. Tuk, whose descriptions, however, cannot be usefully abridged, and are too extensive and detailed for our present space.
Organs of Digestion. - These consist of a simple longitudinal pouch forming the stomach or main intestinal canal, contracted at each end, enlarged in the middle, and giving off on all sides numerous lateral pouches or caeca of different sizes, to the nbumber, in Phalangium opilio (Latr.), of thirty. These are minutely detailed by Tulk, who divides them into - (1), those on the dorsal surface of the main capal; (2), those on its ventral surface; (3), those on its sides. The pharyngeal tube and oesophageal canal are modified so as to be qualified not only for the passage of simple fluids, but also for solid substance; these first, however, having undergone a process of quasi-mastication by means of the external parts of the mouth. At the opposite end of the alimentary a simple, short intestine connects it with the anus, the external aperture of which is situated at the hinder-extremity of the abdomen. With regard to the office of the lateral caeca, an examination of their contents has led to the suggestion that they secrete a peculiar granular substance, which is thence discharged into the stomach, and "agglutinates the particles of food together," covering them with a membranous integument which serves to protect the delicate coats of the intestine from laceration by the sharp and spiny nature of their food-contents (Tulk, l.c. p. 248). Biliary tubes have been described by Treviranus; but it seems probable that the anterior pair of thesem at least, are tracheae connected with the stigmata on each side of the fore margin of the cephalo-thorax. These stigmata were mistaken by Tulk for eyes (l.c. p. 156).
The Organs of Respiration are tracheal. The stigmata in Phalangium (andGonyleptes) are four in number, - tow beneath the fore part of the abdomen, situated one at the base of each of the posterior legs, but usually concealed by the coxal joints, and one on each side of the fore margin of the cephalo-thorax; each of the former pair opens into a large obliquely longitudinal tracheal trunk; these two trunks give off various smaller branches and tubes, which convey the air to all parts of the body. With two of these tubes, probably, the cephalic stigmata are connected.
The Organs of Circulation are very simple, consisting, in Phalangium, of an elongated dorsal vessel acting as a heart, lying in a groove upon the upper surface of the alimentary canal, and divided into three chambers. Ramifications from this vessel serve to carry the vital fluid to every portion of the body (Tulk). From researches since made by M. Blanchard, it appears that this fluid is recollected from the general circulation into two large lateral vessels (vascular sinuses), from which it is returned by other channels to the dorsal vessel, or heart itself. The structure of the heart consists of a series of transverse, curved, and muscular bands, leaving between them light and membranous intervals (Tulk).
The Nervous System of phalangids is very similar in its type to that of some other arachnids. Two large ganglia (cephalic and thoracic) occupy the cephalo-thorax, and from these nerves issue to the various parts, some of them leaving ganglionic enlargements in their course, whence again finer nerves distribute themselves to the adjacent structures. Tulk mentions a striking peculiarity connected with the nervous system of phalangids, which is a power possessed by them to move the nervous centres backwards or forwards at will; this is effected by means of several large transversely striated muscular fasciculi radiating from the sides of the thoracic ganglion, to which they are attached by short tendons (Tulk, l.c. 326).
Thr Organs of Generation in the Phalangidea (which are oviparous) are different from all other arachnids in the large external parts connected with them. In the female, the internal parts consist of an ovisac and ovarium (Tulk); whence, by means of the oviduct, the ova are conducted into the ovipositor. This is a long membranous tube; the first or basal part of it is surrounded by annuli or rings of hairs or bristles; towards the extremity it is scaly, and also furnished with some hairs, and the extreme point has two small lateral tufts of hairs. The ovipositor is protruded, at the will of the animal, from the vulva, which is situated at the fore margin of the sternal, plate, or else it is, when at rest, withdrawn into its sheath, leaving no outward trace of it visible. In the male, the seminal vessels consist of a cluster of "slightly tortuous caecal tubes," whence the spermatic fluid is passed by at duct into a penis of enormous length compared to the size of the animal (fig. 12, de, e,f). This organ is a slender curved tube furnished at its extremity with a recurved hook, and contained in a sheath; both the sheath and penis when at rest lie in a longitudinal direction beneath the ventral surface of the thorax, but can be protruded at will from the external aperture, which is similar, and similarly situated, to that of the female. The form of the penis varies in different species, as also does the ovipositor of the female. The organs of generation, both male and female, of Phalangium oplio, are detailed at great length by Tulk, l.c. Both the penis and ovipositor may be made to protrude by a slight lateral pressure beneat h the fore margin of the sternal plate.
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Arachnida - Table of Contents