Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/297

Rh PYCXOGOXIDS.j ARACHNIDA 277 though their true position has been hitherto (and to some extent still is) a matter of much difference of opinion among naturalists. They seem to connect the more typical Arachnida with the Crustacea, and also to form a passage from the Acaridea to the Pkalaiigidea, It appears, however, when their peculiar structure, both external and internal, is considered, impossible to include them within either of those orders ; it is therefore proposed here to constitute them a separate order between the acarids and phalangids. The characters of the order arc EXTERNAL STRUCTURE. B *dy, linear, composed of four segments ; caput, tubular, in form of a beak or proboscis, and either simple, or ending with palpi and mouth organs ; abdomen, rudimentary ; leys, eight (each thoracic segment bearing a pair), very long, and consisting of eight to nine joints, terminating with a claw or claws. In the immature (so-called larval) state the legs are four only in number. The females have a supernumerary pair between those of the first pair, used for carrying the ova. Palpi, when present, filiform, and composed of five to nine joints ter minating with a claw. Eyes, four. With regard to their internal structure, the Pycnogonidea have a dorsal vessel (circulatory apparatus) divided into chambers, a stomach sending forth caeca! prolongations into the legs and palpi, an abdominal intestine with dilated cloaca, and a nervous system, consisting of a cephalic and opththalmic ganglia, closely united together and joined by the cesophagian fillet to the abdominal ganglia, which are four in number, of large size, sessile, and emitting from their lateral extremities nerves into the legs (Cuvier s Itign. An., edition cited post, pi. 22), but no organs of respiration (Huxley, I.e., and Cuvier s Rcyn. An., I.e.) Fig. 8. Flo. l.rytnogonum littoraJe, Mttll^r. a, parts of mouth, forming a beak, 6 caput ; c...c, thoracic segments; c/, rudimentary abdomen ; t, eyes. FIG. H. The same ; under side, a, a, supernumerary pair of leg. This order contains but one family, Pijcnogonidcs, divided into peveral genera Pycnogonum, Phoxiehilus, Nymphon, and Ammo- th3i. borne of the species arc parasitic on cetaceous animals, others are found secreted among seaweed along the sea-shore, and feed on small marine animals. Some are British, and others exotic. The genera may be thus shortly characterised. Genus Pycnogonum (Brunnich). Body short and thick ; logs, short, strong; without either falces or palpi. The species of this genus are parasitic on thecetacea. Figs. 7, 8, P. Uttoralc (Miiller). Genus Phoxichilus (Latr.) Body narrow; legs of great length ; falces present, but no palpi. Genus Nymphon (Fabr. ) -Resembles Phoxkhilus in the narrow body and long legs, which arc also slender, as well as in hav ing falces; but in the present genus there are also two palpi, each of five joints. Fig. 9, Xymphon coccineuni (Johnston). Genus Ammothca (Leach). Body short and rather broad ; legs long ; beak of great size and length ; much longer than the falces, which arc short. Pal] i 9-jointed, the third joint much the longest ; terminal tarsol claws double, and of unequal size. Supernumerary legs 9- , i i ,1 i ,1 tic. 9. aytnphon coccintum. Jo ns on : jointed, and inserted under the UIld er s.Ue. a, la-ud ; 6, !&amp;gt;, ia!pi. tirst pair of legs behind the beak. The latest authority in regard to their systematic position, i.e., whether Crustacean or Arachnidan, is Dr Anton Dohrn, who says in a memoir, &quot; Ueber Entwicklung uud Bau der Pycnogouiden &quot; (contained in a publication, entitled Untersttchuiyjen tiler L au iind Enticickhing dcr Arthropodcn, Leipsic, Engleman, 1870) : &quot; Die PycnogoniUen sind weder Arachnideu noch Crustaceen.&quot; Dr Dohrn enters fully into the development and structure of Pycno- fionum littoraJe, and of a species of another genus formed out of Phoxichilus, Achclia Iccvis. The present writer has not, however, had an opportunity of studying this memoir. Order III. PHALAXGIDEA. The Phalangidea present a very well defined and compact group, though the transition, in one plane, from the Acaridea, through the families Cyphophthalmides and Trogulides, is tolerably apparent, while in another plane they show evident affinity to the Pycnogonicles. The following are more detailed characters of the order : EXTERNAL STRUCTURE. The body is of a more or less round, oval, or oblong, and sometimes quadrangular form; the integument is either hard and horny, or coriaceous, and generally destitute of hairs, but often tuberculous and armed with spines. The ceplialo-thorax and abdomen are united throughout their whole breadth, the junction frequently shown by a slight constriction, but always visible from a strongly- marked groove or ridge ; the upper side of the cephalo- thorax, and sometimes of the abdomen, is frequently furnished with spines, sharp teeth, or tubercles. The abdomen is articulate, or segmented in a modified form, from several more or less strongly developed transverse foldings in the epidermis. The mouth organs are rather complex (fig. 11), consisting of three pairs of maxillae, above which is a tongae(langvftte) ; and below tho maxilla) is a membranous lip (labivm). From the first pair of these maxillae there issue two five- jointed palpi, the intermediate joints of which often exhibit processes or branches, the last or digital joints ending with a claw. The other two pairs of maxillae support the basal joints of the two first pairs of legs. The presence of these supernumerary, or crural, maxilla? strongly supports the idea long since advanced by Savigny and others, that the palpi are but modified legs (see, however, note 1, p. 272 supra); the same idea being also raised in regard to the Thely- phonidea and Solpugidea by the use as palpi of the first pair of legs, which are in fact completely palpiform, and differ markedly from the rest in their structure. Above these parts, directly in front of the upper extremity of the cephalo-thorax, are two fulees, each of which consists of two, or (in Cyphophthalmides, G. Joseph) three joints;