262 DR- C. CHILTON ON THE SUBTERRANEAN 



than their surface-dwelling allies. In some cases the increased length of the antennae, 

 &c., would no doubt give greater tactile power ; but it also appears probable that, in the 

 case of species living in underground waters, the greater slenderness of the body has 

 been acquired in order to adapt the animal for its life in the restricted spaces betw een 

 the stones and shingle in which it has to live. This explanation is strongly suggested 

 by a comparison of the two subterranean species of Phreatoicm with the surface-species 

 P. australis. 



The whole subject of the compensation for the loss of eyesight has been fully 

 discussed by Packard [83, pp. 123-130], who has supplemented the anatomical descrip- 

 tions with an account of what is known of the habits of some of the species. To this 

 account I must refer the reader, and I shall only add here the few facts bearing on the 

 same question that I have observed in the New-Zealand specie s. 



In the two species of Phreatoiciis, P. typicus and P. assimUis, the body is more slender 

 and the antennae and legs longer, especially in P. typicus, than in the surface-species 

 P. ausiralis, but I have not observed any marked increase in the number or size of the 

 olfactory rods, nor have I observed any other sensory setae like those found in some of 

 the other species. 



In Cruregens fontanus the antennae are well supplied with olfactory rods, which 

 appear to be more numerous and of greater length than in allied eyed species ; besides 

 these, other sensory setae, like the " soies auditives" described by Sars, are found in 

 considerable abundance on the antennae, the various joints of the legs, and the urojioda. 

 Similar setae are found in the same places in eyed species of AntJiura, Paranthura, &c., 

 and Sars has figured them also in Asellus aquaticus, but they are rather more numerous 

 and are longer in Cruregens than in the other species . 



In Crangonyx compacfns I have not noticed any increase in the number of sensory 

 setae beyond what we usually find in similar Amphipods ; in this species, too, the body 

 is compact and not elongated as in some other species, and the legs are only of the usual 

 length. 



In Gammarus fragilis the body is slender, and the antennae, peraeopoda, and terminal 

 uropoda are much elongated. On the peraeopoda, the last three pairs of which are of 

 great length, setae somewhat like the " soies auditives " occur in most of the tufts 

 of setae fovmd on the various joints, but with this exception there does not appear to be 

 any increase of sensory setae beyond what we usually find in species of Gammarus. 



In Calliopms suhterraneus the body is rather more slender and the various limbs 

 rather more elongated than in the closely allied species Plierusa ccerulea. Numerous 

 " soies auditives " are found at various places on the antennae, which, in the male, are 

 also abundantly suj)plied with " calceoli," which are no doubt also sensory in function. 

 These, however, are also found in Plierusa cmrulea and in CalUoplus JiiwiatiUs, and in 

 some species described by Stebbing, such as Eusiroicles Ccesaris, &c., the calceoli are 

 quite as numerous as in Calliopius suhterraneus. 



On the whole the New-Zealand subterranean Crustacea give only a modified support 

 to the conclusion that subterranean species are more abundantly supplied with sense- 

 organs (other than eyes) than allied surface-animals. Probably the former have been 



