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(Milne-Edwards, 1837)

Description:
? Generic features. The structure of the thoracic limbs is are markedly more unguiculate than in any other genus of mysids. The claw is surrounded by a stiff brush of long, peculiar, microscopically spinose setae, giving a very characteristic appearance which can readily be recognised.
Carapace rather short. Antennal scale with outer margin naked and ending in a thorn, apex obliquely truncate. Eyes well developed.
Labrum much longer than broad, produced anteriorly into very long process.
Mandible molar tubercle feebly developed; maxillae narrow, lobe from the third segment feebly cleft.
Proximal segment of exopod of uropod armed with spines but no setae. Endopod of uropod undivided.
Thoracic endopod markedly unguiculate. First pair of thoracic limbs slender, gnathobasic lobe on the second segment only of the endopod or entirely absent; third pair of thoracic limbs with the endopod not elongated and dactylus and claw well developed; pleopods of female reduced to small, simple, linear plates with a few plumose setae. Exopod of uropod with distinct distal suture. Telson entire.
Three pairs of oostegites in the female.
? Species. Body very long and slender, not tapering markedly posteriorly. Rostrum very long and acutely pointed, reaching almost to the distal end of the second segment of the antennular peduncle. Exopod of uropod with a distinct distal suture. Antennal scale with outer margin naked and ending in a thorn. Telson entire. Thoracic endopods markedly unguiculate.
Carapace small, leaving the last three thoracic somites almost completely uncovered.
Antennal peduncle very long and slender, more than half the length of the carapace, first segment longer than the second and third together, distal segment in the male with very large hirsute lobe; in the female the inner margin of this segment is furnished as a rule with four long, plumose setae in addition to the two or three at the distal inner corner.
Antennal scale long and narrow with the sides almost parallel, outer margin naked and slightly concave, inner margin setose and slightly convex, unarmed portion of the outer margin terminating in a strong tooth beyond which the apex is produced and cut off obliquely, a small distal suture present.
Mandibles with molar tubercles feebly developed.
Eyes with long cylindrical stalks projecting laterally considerably beyond the sides of the carapace; cornea broader than stalk; pigment very black.
First thoracic leg small with inconspicuous nail. Second thoracic leg longer than first with merus and carpus subequal. Third to the eighth thoracic legs very slender, carpus and propodus equal in length, dactylus short, nearly square, forming with the nail a long slender claw surrounded by a dense brush of stiff, microscopically spinous setae which spring from the distal end of propodus.
Telson long and slightly tapering, entire; proximal marginal spines subequal, distally the large prominent spines have small ones between, arranged in series of six to ten and very crowded; apex narrowly rounded, with two large spines, and between them from three to five (usually four) small equal spinules and two small setae.
Uropods very long and slender; exopod from six to six and a half times as long as broad, with a distal suture cutting off about one-sixth of its length. Outer margin, from near the proximal end back to the suture, armed with a continuous row of about thirty spines which increase regularly in size distally; the proximal ones may show a tendency to be arranged in series. Distal segment about one and a half times as long as broad. Endopod shorter than, and about half as broad as, the exopod, tapering distally, armed along the inner margin with a row of spines which increase in size distally.

Colour:
Almost completely transparent, lacking all trace of chromatophores on the posterior part of the thoracic somites: sometimes light brown becoming reddish towards the posterior end.

Size:
Up to 21 mm.

Habitat:
Hyperbenthic; shallow waters: 0-30 metres. Rarely taken at a depth of more than 20 metres.

Remarks:
This species is definitely a northern form, being found abundantly around the British Isles and along the north coast of France.
Siriella armata lives in swarms, swimming among weeds at low-water mark, and in clear pools with rocky bottoms or under overhanging rocks where there is mud. Normally it lives floating idly just clear of the bottom, but during the breeding season the animals migrate to the surface at night and may be taken there in townets. The species forms part of the food of cod in the southern part of the North Sea.

Distribution in the North Sea:
All North Sea, shallow waters.

World distribution:;
E North Atlantic: 33-56°N; Mediterranean; coastal, shelf.

Siriella armata