68 DR. J. D. MACDONALD ON TANAIS VITTATUS. 



articulation (fig. 2, c) represents the so-called olfactory organ of the Decapod. The second 

 or inferior pair of antennae are narrower than the superior, and composed of five joints, 

 of which the first, third, and fifth are shorter than the second and fourth ; and the ulti- 

 mate joint is tipped with a brush of long and simple hairs, springing from what would 

 appear to he an aborted multiarticulate filament. 



The oral organs consist of a spatulate upper lip, fringed with short hairs (fig. 6, a), a 

 pair of two-branched mandibles without palps (b), and articulated laterally with the buccal 

 frame, two pairs of maxillae (c & cl), and a pair of five-jointed jaw-feet (/), inclosing, at 

 their base, two membraneous plates, forming a lower lip (e) . 



The internal maxilla? (c) are in the form of rounded membraneous lamellae, fringed 

 with fine hairs, and presenting a single-jointed palpiform process near their summit 

 externally. 



The external maxillae (d), on the contrary, are strong and sabre-shaped, with a spiny 

 and setaceous extremity. Two oval leaflets (d'), connected with these internally, form a 

 tongue-like organ in the mouth ; and at their base externally, but concealed by the cara- 

 pace, is a conical appendage (d") or flagellum, with long diverging hairs at the tip. In 

 the possession of this appendage, as Fritz Muller has pointed out, the Tanaidae make a 

 still further approach to the Decapoda. Thus, in his ' Facts and Arguments for Darwin '* 

 he remarks, "Whilst in all other Oniscoida the abdominal feet serve for respiration, 

 these in our cheliferous Isopod (Tanais dubms(?) Kr.), are solely motory organs, into 

 which no blood-corpuscle ever enters ; and the chief seat of respiration is, as in the Zoeae, 

 in the lateral parts of the carapace, which are abundantly traversed by currents of blood, 

 and beneath which a constant stream of water passes, maintained, as in the Zoeae and the 

 adult Decapod, by an appendage of the second pair of maxillae, which is wanting in all 

 other Edriophthalmata." In passing, I may mention that the figure of Tanais dubms, so 

 called, loo. cit., is rather more conformable to the genus Leptochelia. In this connexion 

 Bate and Westwood's beautiful work on the British sessile-eyed Crustacea may be con- 

 sulted with advantage. But to return to the subject, the joints of the jaw-feet (f) are 

 furnished with rigid comb-like bristles, arising from the inner border of the third and 

 fourth, and the free extremity of the fifth. The lower lip (e) is also fringed with fine 

 short hairs. 



Following the jaw-feet just noticed are the seven pairs of legs common to all the Edri 

 ophthalmata, the first two and last five respectively corresponding with the two external 

 pairs of jaw-feet, and the five pairs of legs appertaining to the thorax of the Decapod. 



The two first limbs (fig. V) are strongly chelate, and many times thicker than the 

 ambulatory legs succeeding them. The fixed claw presents a trenchant upper border, 

 more prominent in the middle, with which, at least in the female, the movable claw cor- 

 responds more or less closely. In the male, however, (fig. 15) both claws are more 

 arcuate. As in the beak of the Crossbill, the opposable tips of the claws are curved 

 with sufficient obliquity to cross over one another, the index taking the outer side. They 

 are of a rich reddish amber tint, similar to that of the dental processes of the mandibles. 



* Translated by W. S. Dallas, F.L.S. 



