Glands.—In some or all of the appendages of the Pycnogonida may be found special glands with varying and sometimes obscure functions. The glands of the chelophores (Fig. [280], p. 522) are present in the larval stages only. They consist of a number of flask-shaped cells[[398]] lying within the basal joint of the appendage, and generally opening at the extremity of a long, conspicuous, often mobile, spine (e.g. Ammothea (Dohrn), Pallene, Tanystylum (Morgan), Nymphon brevicollum and N. gracile (Hoek)). They secrete a sticky thread, by means of which the larvae attach themselves to one another and to the ovigerous legs of the male parent. In Nymphon hamatum, Hoek, the several filaments secreted by the separate sacculi of the gland issue separately. In Pycnogonum the spine on which the gland opens is itself prolonged into a long fine filament, and here, according to Hoek, the gland is in all probability functionless and rudimentary. Hoek has failed to find the gland in Ascorhynchus, and also in certain Nymphonidae (e.g. Boreonymphon robustum, Bell), in which the young are more than usually advanced at the time of hatching. The gland has also been described by Lendenfeld and others in Phoxichilidium, whose larvae do not cling together but live a parasitic life; in this genus the long spine or tubercle is absent on which the orifice is usually situated, and, according to Lendenfeld, the secretion issues from many small orifices set along the opposing edges of the chela. Of the two species described by Dohrn as Barana castelli and B. arenicola, the former has the spine of inordinate length, more than twice as long as the whole body, chelophore and all; while in the latter (which species rather resembles Ascorhynchus) the spine is altogether absent.
In the palps and ovigerous legs of the adult are found glandular bodies of a hollow vesicular form with a simple lining of cells, the vesicle being divided within by a septum with a central orifice, the outer and smaller half opening to the exterior. These glands are probably of general occurrence, but they have been but little investigated. They lie usually in the fourth and fifth joints of the palp, and the third and fourth joints of the ovigerous leg. Hoek describes them in Discoarachne (Tanystylum) as lying within the elongated third joint of the palp, and opening by a sieve-plate at the end of the second joint. In Ammothea (Dohrn) and Ascorhynchus (Hoek) they open on a small tubercle situated on the fifth joint of the palp. In Nymphon, Hoek describes them as opening by a small pore on the fourth joint of the ovigerous leg. Dohrn failed to find them in Pycnogonum, but in Phoxichilus, Phoxichilidium and Pallene he discovered the glands appertaining to the palps, though the palps themselves have disappeared in those genera; he has found the glands also in Ammothea, in larvae that have not yet attained their full complement of legs.
The males in nearly all cases are known to possess glands in the fourth joints or thighs of all the ambulatory legs, and these glands without doubt act as cement-glands, emitting, like the chelophoral glands of the larvae, a sticky thread or threads by which the eggs and young are anchored to the ovigerous legs. In some species of Nymphon and of Colossendeis Hoek could not find these, and he conjectures them to be conspicuous only in the breeding season. While in most cases these glands open by a single orifice or by a few pores grouped closely together, in Barana, according to Dohrn, and especially in B. arenicola, the pores are distributed over a wide area of the femoral joint.[[399]] In Discoarachne (Loman) and Trygaeus they open into a wide chitinised sac with tubular orifice. While the function of these last glands and of the larval glands seems plain enough, that of those which occur in the palps and ovigerous legs of both sexes remains doubtful.
In their morphological nature the two groups of glands are likewise in contrast, the former being unicellular glands, such as occur in various parts of the integument of the body and limbs of many Crustacea; while the latter are segmentally arranged and doubtless mesoblastic in origin, like the many other segmental excretory organs (or coelomoducts) of various Arthropods.
By adding colouring matters (acid-fuchsin, etc.) to the water in which the animals were living, Kowalevsky demonstrated the presence of what he believed to be excretory organs in Phoxichilus, Ammothea, and Pallene. These are small groups of cells, lying symmetrically near the posterior borders of the first three body-segments, and also near the bases of the first joints of the legs, dorsal to the alimentary canal.[[400]]
Fig. [277].—Longitudinal section through one “antimere” of the proboscis in Phoxichilus charybdaeus. G, g′, Principal and secondary ganglia; h, sieve-hairs; L, lip; mt, oral tooth; N, N′, inner and outer nerve-cords; t, proboscis-teeth. (After Dohrn.)
Alimentary System.—The proboscis is a very complicated organ, and has been elaborately described by Dohrn.[[401]] It is a prolongation of the oral cavity, containing a highly developed stomodaeum, but showing no sign of being built up of limbs or gnathites. The mouth, situated at its apex, is a three-sided orifice, formed by a dorsal[[402]] and two lateral lobes; and hence the proboscis has been assumed by some, on no competent evidence, to be constituted of a degenerate pair of appendages and a labrum or upper lip. Each of the three lobes which bounds the mouth shows the following structures: firstly, a lappet of external chitinised integument, overlapping, as the finger-nail overlaps the finger, a cushion-like lip, ridged after the fashion of a fine-cut file in some species, hairy in others, on the inner surface where the three lips meet to close the orifice of the mouth. Below this again is a prominent tooth (Fig. [277], mt), supported, as are the lips, by a system of chitinous rods, which are but little developed in the genus here figured, though conspicuous and complicated in others. Transverse ridges run across the angles where adjacent lips meet, and the whole mechanism constitutes an efficient valve, preventing the escape of swallowed food. The greater portion of the proboscis is occupied by a masticating or triturating apparatus, the oesophageal cavity expanding somewhat and having its walls densely covered, in three bands corresponding to the antimeres, with innumerable minute spines (h) or needles, sometimes supplemented by large teeth (t) that point forwards somewhat obliquely to the axis of the proboscis.[[403]]
In the curious East Indian genus Pipetta (Loman) the sucking and sifting mechanism is low down in the proboscis, and the organ is prolonged into a very fine tube, the lips growing together till they leave an aperture of only ·007 mm. for the absorption of liquids.
In some cases, where the proboscis itself is short, as in Pallene, this mechanism is carried backwards into the fore-part of the body; and, in the latter genus, the narrow oesophagus which succeeds the masticatory apparatus is likewise provided with extrinsic muscles.