The eye of a Pycnogon (Phoxichilidium) is composed of three layers, an outer layer of specialised ectoderm cells (hypodermis) that secrete the cuticular lens, a middle layer of visual or retinal elements, and an inner layer of pigment-cells. The elements of the middle layer consist of much elongated cells, whose branching outer ends are connected with nerve-fibrils and interwoven in a protoplasmic syncytium, whose middle parts are occupied by the nuclei and whose inwardly directed ends form the retinal rods or bacilli. The pigment-cells of the inner layer are of various forms, those towards the middle of the eye being small and flattened, those at the sides being, for the most part, long and attenuated, so seeming, as Morgan remarks, to approximate in character to the retinal elements. The pigment-layer is easily dispersed and reveals beneath it a median vertical raphe, caused by the convergence of the cells of the middle layer from either side, and along the line of this raphe the optic nerve joins the eye, though its subsequent course to its connection with the retinal elements is obscure. It is at least clear that the retina is an “inverted” retina, with the nerve-connected bases of its cells lying outwards and their bacillar extremities directed inwards.
In a longitudinal vertical section of the eye of a larva (Tanystylum), at a stage when three pairs of walking legs are present, Morgan shows us the pigment-layer apparently continuous with the hypodermis just below the eye, and in close connection with the middle layer at the upper part of the eye. From this we are permitted to infer a development by invagination, in which the long invaginated sac is bent and pushed upwards till it comes into secondary contact with the hypoderm, so giving us the three layers of the developed eye. This manner of formation is precisely akin to that described by Parker, Patten, Locy, and others for the median eyes of Scorpions and of Spiders, and the organ is structurally comparable to the Nauplius- or median eye of Crustacea. But neither in these cases nor in that of the Pycnogon is the whole process clear, in consequence chiefly of the obscurity that attends the course of the optic nerve in both embryo and adult. For various discussions and accounts, frequently contradictory, of these phenomena, the reader is referred to the authors quoted, or to Korschelt and Heider’s judicious summary.[[406]]
There seems to be a small structure, of some sort or other, between the ocelli on either side. Dohrn thought it might be auditory, Loman that it might be secretory, but its use is unknown.
Integument.—The chitinised integument is perforated by many little cavities, some of them conical and tapering to a minute external pore, the others more regularly tubular. Sometimes, but according to Hoek rarely, the tubular pore-canals communicate with, or arise from, the conical cavities. The pore-canals transmit a nerve for the supply of sensory hairs, often forked, which arise from the orifice of the canal in little groups of two or more, sometimes in rosettes of eight or nine. These setae are small or rudimentary in Ascorhynchus and totally wanting in Colossendeis; they appear to be extremely large and stellate in Paranymphon. The conical cavities contain proliferated epithelial cells, blood corpuscles, and cells of more doubtful nature that are perhaps glandular. According to Dohrn, glands exist in connection with both kinds of integumentary perforations, and he suspects that they secrete a poisonous fluid in response to stimuli affecting the sensory hairs; Hoek, on the other hand, is inclined to ascribe a respiratory function to the cavities; but indeed, as yet, we must confess that their use is undetermined.
Reproductive Organs.—In each sex the generative organs consist of a pair of ovaries or testes lying above the gut on either side of the heart; in the adult they are fused together posteriorly at the base of the abdomen, and send long diverticula into the ambulatory legs. In the female Phoxichilidium, at least, as Loman has lately shown, the fusion is complete, and the ovary forms a thin broad plate, spreading through the body and giving off its lateral diverticula. The diverticula of the testes reach to the third joint of the legs, those of the ovaries to the fourth, or sometimes farther. The ova ripen within the lateral diverticula, chiefly, and sometimes (Pallene) exclusively, in the femora or fourth joints of the legs,[[407]] which, in many forms, are greatly swollen to accommodate them; the spermatozoa, on the other hand, are said to develop both within the legs and within the thoracic portions of the testis. The genital diverticula may end blindly within the leg, or communicate through a duct with the exterior by a valvular aperture placed on the second coxal joint. Such apertures occur, as a rule, on all the legs in the females, in Rhynchothorax and Pycnogonum on the last only. In the males an aperture is present on all the legs in Decolopoda and Phoxichilidium; on the last three in Nymphon and Phoxichilus; in most genera on the last two; in Pycnogonum and Rhynchothorax on the last only.
Very commonly the female individuals are somewhat larger than the males, and in some species (Ammothea, Trygaeus) the latter are distinguished by a greater development of spines or tubercles on the body and basal joints of the legs (Dohrn).
The act of fecundation has been observed by Cole[[408]] in Anoplodactylus. The animal reproduces towards the end of August. Consorting on their Eudendrium (Hydroid) colony, the male climbs upon the female and crawls over her head to lie beneath her, head to tail; and then, fertilisation taking place the while, the hooked ovigerous legs of the male fasten into the extruding egg-masses and tear them away. The whole process is over in five minutes. The fresh egg-masses are more or less irregular in shape, and white in colour like little tufts of cotton.
Each ball of eggs that the male carries represents the entire brood of one female, and in Phoxichilidium Loman has seen a male carrying as many as fourteen balls. Fertilisation is external, taking place while the eggs are being laid. The spermatozoa have small rounded heads and long tails, and are thus unlike the spermatozoa of most Crustacea.
Development.—Until the hatching of the embryo, the eggs of the Pycnogons are carried about, agglutinated by cement-substance into coherent packets, on the ovigerous legs of the males. They are larger or smaller according to the amount of yolk-substance present, very small in Phoxichilidium and Tanystylum (Morgan), where they measure only ·05 mm. in diameter; larger in Pallene (·25 mm.); larger still (·5–·7 mm.) in Nymphon. In Pallene each egg-mass commonly contains only two eggs; in the other genera they are much more numerous, rising to a hundred or more in Ammothea (Dohrn). The egg-masses may be one or more on each ovigerous leg, sometimes (Phoxichilidium angulatum, Dohrn) a single egg-mass is held by both legs; they are extremely numerous in Phoxichilus, and in Pycnogonum they coalesce to form a broad pad beneath the body. The fact that it is the male and not the female that carries the eggs was only announced in 1877 by Cavanna;[[409]] before, and by some even after his time, the two sexes were constantly confused.[[410]]
