54 THE VOYAGE OF H.M.S. CHALLENGER. 



that compresses and holds down the posterior margin of the carapace ; the short fur on 

 the post-orbital regions, extending on each side of the gastric region of the carapace, 

 and repeated on the third and fourth somites of the pleon ; and the strength, and more 

 particularly the form of the several parts of the rhipidura or. tail-fan, which is formed 

 by the sixth pair of pleopoda and the telson. 



The sixth pair of pleopoda is implanted on the ventral surface within the margin of 

 the coxal plate and is directed anteriorly, and the outer plate is very much longer than 

 the inner, and possesses a diaeresis near its distal extremity. Although the outer or 

 anterior plate is very much longer than the posterior, yet the latter equals the length of 

 the posterior margin of the former, and their distal margins form a continuous line. The 

 telson is quadrate, and the terminal or distal margin when depressed a little forwards is 

 continuous with the posterior margin of the rami of the sixth pair of pleopoda, and this 

 is capable of resting through its entire length against the floor on which the animal lies, 

 and so enables it to creep backwards with considerable persistence and power. 



The large chelate pereiopoda differ on each side ; each is furnished with long delicate 

 spine-like teeth, but that on the left shows most probably the character and appearance 

 of the normal chela, while that on the right exhibits an extensive deviation. The 

 propodos has increased in size as a consequence of the powerful muscles necessary to sustain 

 and carry the enormously long dactylos and pollex, which nearly equal the entire length 

 of the animal ; the form of the chela is that of two combs meeting, and it appears probable 

 that when partially closed it has the power of raking the neighbourhood to a considerable 

 distance, and so entrapping small animals and other material from which the blind 

 creature has the power of selecting its food, which it carries to its mouth by means of its 

 smaller chelate pereiopoda, the larger ones from their length being incapable of that 

 office. The mouth is furnished with a pair of powerful denticulated mandibles, that are 

 evidently capable of crushing tolerably hard substances. The anterior lip is calcified, 

 firm, and denticulate on the antero- external margin (PL VI. fig. c). The epistoma 

 is horizontal or nearly so, and occupies a considerable space between the antennae 

 and the mouth, separating the phymacerite to a considerable distance from it. The 

 phymacerite is very large, and is situated on the first or coxal joint of the second or 

 outer pair of antennae, which is short, broad, and separated from the body by a 

 distinct suture. 



This animal is intermediate in character between TJialassina and the Astacidae, to 

 which latter family Willemoes-Suhm first referred it under the name of Astacus zaleucus. 

 Its nearest congener appears to be Calocaris, Bell, of the British seas, from which it differs 

 in the third pair of pereiopoda being minutely chelate instead of monodactyle, and in 

 having no apparent organs of vision, instead of, as in Calocaris, having the " eyes rudi- 

 mentary, sub-globose, without any pigment or cornea," which, when Bell described it in 

 1853, was a feature " unique in the whole of the higher forms of Crustacea." Calocaris 



