Marine Arthropoda with a body composed of a cephalothorax, three free thoracic somites, and a rudimentary abdomen. The cephalothorax consists of a tubular proboscis with a terminal mouth, a prae-oral, two post-oral head somites, and one coalesced thoracic somite. It bears four pairs of appendages, three of which belong to the head, viz. a pair of 2-3 jointed prae-oral mandibles often terminated by a chela, a pair of slender palpi, of ovigerous legs usually ten-jointed, and the first pair of thoracic limbs which, like the other three pairs, are eight-jointed and borne upon processes of the somite. The limbs have a terminal and often two accessory claws. The mandibles may be absent, or the palpi, or both, and the ovigerous limbs may be present only in the male, but there is some variety even among specimens of the same genus. The abdomen is rarely segmented and is connected to the thorax by a joint. It bears the anus terminally. The integument contains numerous glands. The supra-oesophageal ganglion innervates the mandibles and eyes, which are placed dorsally on a tubercle of the cephalothorax. The infra-oeso-phageal (=1st thoracic) ganglion innervates the palpi and ovigerous legs, and is composed of two ganglia fused.

There are four well-developed thoracic ganglia, and two others which are either rudimentary or aborted. The species inhabiting shallow water have four eyes, those living below 400 fathoms rudimentary eyes without pigment, or none at all. The eyes appear to be monostichous and monomeniscous. The alimentary canal consists of an oesophagus dilated posteriorly, and containing a masticatory apparatus; of a mesenteron beset with cellular villi (= glands) and giving off caeca which enter the proboscis in many instances, the mandibles and limbs, but not the palpi and ovigerous limbs. Their number is not regular apparently. There is a dorsal heart which is continuous dorsally with the integument, open anteriorly and furnished with two pairs of lateral ostia, and either a third pair or a single posterior aperture. Respiration is cutaneous, and there are respiratory cavities or depressions opening externally by a pore. The testis consists of two dorsal tubes united posteriorly, and sending processes outwards into all the thoracic limbs, or into the three or the two last pairs only. The ducts open externally by a ventral pore on the tibial joints. The ovary either resembles the testis, or the portion within the body becomes atrophied.

The branches in the limbs open in the same position as do the branches of the testis, but there are no true ducts. Copulation takes place. The eggs are fecundated externally, and are carried by the male on the ovigerous limbs, by the female also in Nymphon brevicaudatum. They are attached by the secretion of special glands placed in the fourth joint of the limbs. The three pairs of cephalic limbs are formed as a rule first, and the young may be hatched in this condition. They remain attached to the ovigerous limbs by a thread secreted by a gland lodged in the basal joint of the mandible. The larvae of Phoxichilidium are entoparasitic in various Hydrozoa. The affinities of the group are doubtful.

The terms cephalothorax, mandible, palpi, etc. must not be taken as implying a homology with the parts so named in Arachnida or Crustacea.

Hoek, Challenger Reports, iii. 1880; Dohrn, Fauna und Flora des Golfes von Neapel, iv. 1881.