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



pair of pereiopoda, where they are usually small aud sometimes rudimentary. They exist 

 attached to all the pereiopoda except the posterior, and are four in number. The 

 arthropoda are similarly persistent, those attached to the anterior appendages being less 

 developed than those situated posteriorly. There are eight in all — four anterior and four 

 posterior plumes. There are also four pleurobranchise, but these are implanted between 

 the somites above the four posterior pereiopoda, the last being the only plume connected 

 with the posterior pair of pereiopoda. There are none above the first pair. 



The branchial arrangement therefore differs from that of the Scyllaridse, PalinuridaB, 

 or Astacidse. 



In the posterior division or pleon the several appendages have a tendency to vary but 

 little, but that little appears to be fixed and reliable. The most conspicuous and con- 

 venient feature for observation exists in the arrangement of the large dorsal median 

 elevation. In Polycheles crucifera every somite except the first and last has two large 

 teeth, one before the other. 



In Stereomastis suhmi there are also two, but the posterior is much smaller than the 

 anterior. In Penlacheles obscura there are also two on each somite, but these take the 

 form of tubercles rather than teeth. In Pentacheles auriculata the teeth on the third 

 and fourth somites are very long and slender ; in most of the others they are more 

 regular, but vary in number and proportion. Throughout all the species they are 

 invariably directed forwards, a feature that is rare in other forms, and is suggestive of the 

 idea that the animal normally creeps backward. 



The first pair of pleopoda (PL XIV. fig. 1 p, ?) is small, slender, and feeble in the 

 female ; in the male (fig. 2 p, t ) it is large, strong, and well developed, evidently adapted 



for a sexual purpose. It is spatuliform and slightly curved, with 

 the convex part towards the ventral surface of the pereion. The 

 broad and curved blade lies anterior to the coxa or first joint of the 

 posterior pair of legs (PL XIX. p $ , o), so that the vas deferens, when 

 projecting as it is capable of doing to a considerable extent (PL XV. 

 fig. 3 o, $ ), falls upon the concave surface of the spoon-like pleopod, 

 and is evidently held by it and directed against the ventral surface 

 of the female, where its extremity rests in contact with or near the 

 vulvar opening. That it does not enter the passage of this latter 

 Pl podl 5 'ma^an'i a feni°ak^xt or g an ma y be assumed from the circumstance that the foramen 

 From ^drawing by wii'ie- through which the vas deferens projects on the fifth pair of legs is 

 much larger than that of the female on the third pair. We may 

 therefore assume that the first pair of pleopoda conducts the extended vas deferens to 

 its position and holds it there. This pleopod is the incipient representative of that organ 

 which we see in the Brachyura, holding the vas deferens in an enclosed tube, and fulfilling 

 the office of an intromittent organ. 



