414 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



This family differs widely from all the other Isopoda, and indeed 

 from all the sessile-eyed Crustacea, in the structure of the respiratory 

 organs, and in the fact that the eyes, when present, are articulated with 

 the head, or stalked, though without any proper pedicel. 



I have seen species of only two genera, Leptochelia Dana and Tanais 

 Audouin and Edwards, from within our limits. These genera are, by 

 some authors, united under the name Tanais^ but there seems to be 

 ample reasons for sei^arating them. While they agree in many charac- 

 ters, they differ widely from Apseudes Leach, which should probably be 

 regarded as belonging to a different family not represented on our coast, 

 and is accordingly not included in the above diagnosis. 



Our representatives of the Tanaidce may be further characterized as 

 follows : The body is subcylindrical and elongated, from four or five to 

 at least eight times as long as broad. The head and first thoracic seg- 

 ment are covered by the large cephalothoracic shield, which tapers 

 somewhat in front, and is dilated behind. Its posterolateral regions 

 are occupied on each side by the branchial cavity, opening behind by a 

 vertical slit, and in front by a nearly horizontal orifice. During life a 

 lash-like organ can be seen through the body wall, in constant vibration, 

 propelling a stream of water from behind forward through the cavity. 

 The eyes, when x)resent, are distinctly articulated with the head, and in 

 the males are generally larger and more coarsely granulated than in the 

 females. They are absent in one of our species, as in the one mentioned 

 by Willemoes-Suhm from 1,400 fathoms in the Atlantic Ocean, off the 

 North American coast, obtained by the Challenger expedition. They are 

 described as indistinct in other foreign species. The antennulae are in- 

 serted close together immediately below the vertex of the head and 

 between the eyes. They are robust at base, and in the males may be 

 elongated, but in the females are short, with only three or four segments 

 and a minute rudiment of a flagellum. In neither sex have they any 

 trace of the secondary flagellum seen in Apseudes. The antennae are 

 more slender than the autennulae, and inserted almost directly beneath 

 them. They are five-jointed, with the first and second segments short, 

 the third larger and longer, the fourth and fifth slender and cylindrical, 

 and, like the antennulae, with indications of a flagellum. The antennae, 

 like the antennulse, are tipped with bristles and bear a few scattered 

 similar bristles on their segments. 



The mouth organs are aborted in the males, at least in the genus Lep- 

 tochelia., but in the females the mouth is protected below by a well-devel- 

 oped pair of maxillipeds, of which the basal segments meet at an angle 

 forming a keel on the under surface of the head. The palpi of the max- 

 illipeds are four-jointed, and armed with strong cilia ; the last segment is 

 strongly flexed on the penultimate. The inner maxillae are spiny, and 

 have the outer lobe reflexed and bearing elongated cilia at the tip. The 

 mandibles are strong, destitute of palpi, and armed with one or two 

 dentigerous lamellae at the apex and a strong molar process. 



