Fig. [280].—Young larva (nat. size ·1 mm.) of Ammothea fibulifera, Dohrn. C.G, Brain; gl, gld, gland and duct of chelophore; pr, proboscis; I, II, III, IV, appendages. (After Dohrn.)
Segmentation is complete, symmetrical in the forms with smaller eggs, unequal in those burdened with a preponderance of yolk (Morgan). In Pallene, as in the Spider’s egg, what is described as at first a total segmentation passes into a superficial or centrolecithal one by the migration outwards of the nuclei and the breaking down of the inner ends of the wedge-shaped segmentation-cells. The blastoderm so formed becomes concentrated at the germinal pole of the egg. A thickened portion of the blastoderm (which Morgan compares to the “cumulus primitivus” of the Spider’s egg) forms an apparently blastoporal invagination (though Morgan calls it the stomodaeum), and from its sides are budded off the mesodermal bands. Meisenheimer has recently given a minute account of the early development of Ammothea, a form with small yolkless eggs. Here certain cells of the uniform and almost solid blastosphere grow inwards till their nuclei arrange themselves in an inner layer of what (so far as they are concerned) is a typical gastrula, but without any central cavity. The inner layer subsequently, but slowly, differentiates into the mid-gut, and into dorsal and lateral offshoots, the sources of the heart and of the muscles and connective tissues respectively. The further development of the egg takes place, as is usual in Arthropods, by the appearance, in a longitudinal strip or germ-band which enwraps the yolk, of paired thickenings which represent the cerebral and post-oral ganglia, and of others from which arise the limbs. Of these latter, the chelophores are the first to appear, on either side of the mouth; in Pallene the fourth pair appears next in order, followed by the fifth and sixth, and by the third and seventh just before the hatching out of the embryo; the second is lacking in this particular genus. Thus in Pallene (Dohrn, Morgan), and in some others, e.g. Nymphon brevicollum (Hoek), the free larva is from the first provided with its full complement of limbs. Certain other species of Nymphon hatch out in possession of four or five pairs of limbs, but in the great majority of cases studied the larval Pycnogon is at first provided with three pairs only, the three anterior pairs of the typical adult.[[411]] Numerical coincidence, and that alone, has often led this “Protonymphon” larva to be compared with the Crustacean Nauplius. In the annexed figure of a young larval Ammothea (Achelia), we see the unsegmented body, the already chelate chelophores (furnished with the provisional cement-glands already described), the other two pairs of appendages each with a curious spine at its base, the gut beginning to send out diverticula (of which the first pair approach the chelophores) but still destitute of the anus (which is only to be formed after the development of the abdomen), the proboscis, and one pair of eyes situated close over the pre-oral ganglia. The subsequent changes are in this genus extremely protracted, and terminate with the loss of the chelae, a process which occurs so late in life that the chelate individuals were long looked upon as belonging to a separate genus, the original Ammothea of Hodge, until Hoek proved their identity with the clawless Achelia.
The developmental history of Phoxichilidium and Anoplodactylus is peculiar. The young larvae have the claws of the second and third appendages hypertrophied to form enormous stiff tendril-like organs, with which they affix themselves to the bodies of Hydroid Zoophytes (Coryne, Eudendrium, Tubularia, Hydractinia, etc.), feeding as the adults do: afterwards losing these elongated tendrils in a moult, they pass into the gastral cavity of the Hydroid; in our native species the larva issues from the Hydroid and begins its independent life at a stage when three pairs of ambulatory legs are present and the fourth is in bud.[[412]] The Phoxichilidium larvae were first noticed by Gegenbaur in Eudendrium,[[413]] again by Allman in Coryne eximia.[[414]] George Hodge made detailed and important observations,[[415]] and showed, in opposition to Gegenbaur, that it was the larva which entered the Hydroid and not the egg that was laid therein.[[416]]
Fig. [281].—Larva of Phoxichilidium sp., showing tendril-like appendages of the larval palps and ovigerous legs. (After Dohrn.)
Moseley has the following interesting note in his Challenger Report:[[417]] “The most interesting parasite observed was a form found in the gastric cavities of the gastrozoids of Pliobothrus symmetricus (West Indies, 450 f.), contained in small capsules. These capsules were badly preserved, but there seemed little doubt that they contained the remains of larvae of a Pycnogonid, so that the deep-sea Pycnogonids, which are so abundant, very possibly pass through their early stages in deep-sea Stylasteridae.... The gastrozoids containing the larvae were partly aborted.”
A Pycnogon larva, doubtfully ascribed to Nymphon, has been found living in abundance ectoparasitically on Tethys in the Bay of Naples.[[418]]
Habits.—Of the intimate habits of the Pycnogons we can say little. Pycnogonum we often find clinging, as has been said, close appressed to some large Anemone (Tealia, Bolocera, etc.), whose living juices it very probably imbibes. The more slender species we find climbing over sea-weeds and Zoophytes, where sometimes similarity of colour as well as delicacy of form helps to conceal them; thus Phoxichilidium femoratum (Orithyia coccinea, Johnston) is red like the Corallines among which we often find it, P. virescens green like the filamentous Ulvae, the Nymphons yellowish like the Hydrallmania and other Zoophytes which they affect. On the New England coast, according to Cole, the dark purple Anoplodactylus lentus, Wilson (Phoxichilidium maxillare, Stimpson), is especially abundant on colonies of Eudendrium, whose colour matches its own, the yellowish Tanystylum orbiculare frequents a certain yellowish Hydroid, and of these two species neither is ever found on the Hydroid affected by the other; while, on the other hand, Pallene brevirostris, whose whitish, almost transparent body is difficult to see, is more generally distributed.[[419]] The deep-sea Pycnogons (Colossendeis, Nymphon) are generally (if not universally) of a deep orange-scarlet colour, a common dress of many deep-sea Crustacea.
The movements of the Pycnogons are singularly slow and deliberate; they are manifestly not adapted to capture or to kill a living prey. Linnaeus accepted from J. C. König the singular statement that they enter and feed upon bivalve shells, “Mytilorum testes penetrat et exhaurit”; but the statement has never been reaffirmed.[[420]]
Loman describes Phoxichilidium as feeding greedily on Tubularia larynx, and especially on the gonophores. It grasps them with its claws, sucks them in bit by bit till the proboscis is filled as far as the sieve, whereupon that part of the proboscis squeezes and kneads the mass, letting only juices and fine particles pass through into the alimentary canal. The lateral caeca and the rectum are separated by sphincter muscles from the stomach; the former are in turn filled with food and again emptied; the contents of the alimentary canal are in constant rolling movement, and the faeces are eliminated by the action of a pair of levatores ani, in round pellets.