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



it is short and sickle-shaped, and sometimes biunguiculate, as in some species of the 

 Alpheidge and Hippolytidas. 



The Fourth Pereiopoda. — The fourth pair of pereiopoda undergoes little change 

 throughout the entire order of the Macrura except in a few of the aberrant Anomura, and 

 in the family of the Eryonidse, in which they are chelate in several of the genera, as in 

 Polycheles, Pentacheles, Eryoneicus, and Willemcesia. It is among the most constant in 

 form and simple in character ; it is absent only in the depreciated forms of Acetes and 

 Lucifer, and even here it is the last to disappear. In the genus Stenopus the propodos 

 is reduced to a multiarticulate condition, but is not enfeebled as in Benthoscetes, Smith, 

 and the dactylos terminates in a biunguiculate extremity. 



The Fifth Pereiopoda. — The fifth pereiopoda is a characteristic pair of appendages 

 in many genera. It is, moreover, functionally an important pair, since it contains the 

 termination of the internal portions of the male organs of generation, the extremity of 

 the vas deferens passing through an orifice in the coxal joints (PI. XV. fig. 3o and PI. 

 XIX. fig. o). In Pentacheles and Willemcesia it is chelate in both males and females. 

 In the Scyllaridse and Palinuridse, Homaridse and Astacidse, it is chelate in the females 

 only ; and appears to be functionally so formed to assist in rupturing the ovisac and 

 liberating the brephalos from the ova. 



In the Trichobranchiata it is frequently simple in the male and chelate in the female. 



In the Dendrobranchiata it is always simple in form, but has a tendency to become 

 long, slender, and enfeebled ; in some genera it is multiarticulate and filamentous, as in 

 Benthoecetes ; in Sergestes it becomes- rudimentary in character, and in Acetes and 

 Lucifer disappears altogether. 



Throughout the Phyllobranchiata it is formed on the same general plan as that of the 

 fourth pair, but varies in some genera in having the dactylos short ; it is generally 

 simple, but there are many genera in which the dactylos is reduced to a minute con- 

 dition and attenuated in form. 



In some genera, as in Diaphoropus, the fifth pair is developed to a very great length, 

 far exceeding that of the preceding pairs. Unfortunately we only know the species of 

 this genus in their young condition, so that although in the specimens of Diaphoroipus 

 versipellis (PL CX VII. fig. 3) it has a form approximating to that of the adult, yet it 

 evidently has to undergo one more change before it reaches the permanent stage. This 

 great size, which is chiefly due to the length of the limb and the diameter of the coxal 

 and basisal joints, appears to belong to other genera, such as Anebocaris and Eretmocaris ; 

 but in the specimens of these genera the appendage is unfortunately wanting, the only 

 part preserved being the large coxal joint (PI. CXLV.). 



In the Phyllobranchiata the great degree of degradation which is seen in some genera 



