Above the stomach are situated the spinning glands, the products of which pass, by seven or more tubules, to the orifice already mentioned on the distal joint of the chelicerae. The abdominal or cement-glands are in the anterior ventral portion of the abdomen. No Malpighian tubes have been found.
The tracheae from the anterior stigmata are directed forward; those from the posterior stigmata backward. Bernard[[336]] has found rudimentary stigmata on the remaining abdominal segments.
The heart is the usual dorsal tube, situated rather far forward, and probably possessing only one pair of ostia. The nerve-cord is a double series of ventral masses, united by transverse commissures. These undergo great concentration in the last stages of development, but in the newly-hatched Chernetid a cerebral mass and five pairs of post-oesophageal ganglia can be distinguished.
There are two peculiar eversible “ram’s-horn organs,” opening near the genital opening. They are said to be present only in the male, and have been taken for the male organs, though other writers consider them to be tracheal in function.
Development.—Some points of peculiar interest are presented by the embryology of these animals, the most striking facts being, first, that the whole of the egg is, in some cases at all events, involved in the segmentation; and, secondly, that there is a true metamorphosis, though the larva is not free-living, but remains enclosed with others in a sac attached to the mother.
At the beginning of winter the female immures herself in a silken retreat, her body distended with eggs and accumulated nourishment. About February the egg-laying commences, thirty eggs, perhaps, being extruded. They are not, however, separated from the mother, but remain enclosed in a sac attached to the genital aperture, and able, therefore, to receive the nutritive fluids which she continues to supply throughout the whole period of development.
The eggs, which line the periphery of the sac, develop into embryos which presently become larvae, that is to say, instead of further development at the expense of yolk-cells contained within themselves, they develop a temporary stomach and a large sucking organ, and become for a time independent sucking animals, imbibing the fluids in the common sac, and arranged around its circumference with their mouths directed towards the centre. Subsequently a second embryonic stage is entered upon, the sucking organ being discarded, and the albuminous matter which the larva has imbibed being treated anew like the original yolk of the egg.
It is an interesting fact that in this second embryonic stage a well-marked “tail” or post-abdomen is formed, and the ganglionic nerve-masses increase in number, a cerebral mass being followed by eight pairs of ganglia in the body and eight in the tail. Subsequently a great concentration takes place till, besides the cerebral mass, only five closely-applied pairs of ganglia remain, corresponding to the pedipalpi and the four pairs of legs. Moreover, the first pair advances, so as to lie on the sides of, and not behind, the oesophagus.
Fig. [224].—Three stages in the development of Chelifer. A, Segmenting ovum; B, embryo, with post-abdomen, maximum number of ganglia, and developing sucking apparatus; C, larva. (After Barrois.)