PINNOTHERIDAE. 



It is difficult to give a common diagnostic for this family. Easily as the characteristic ■ 



representatives are recognizable, there is a gradual shading, through the subfamilies Pinnothere- ^ 



linae and Asthenognathinae^ to the Hexapodinae, which are referable to the Goneplacidae. With \ 



these Hexapodinae the members of the present family also share the commensalistic habits : \ 



the majority of the species, as far as the host is known, inhabit the mantle-cavity of Lamelli- i 

 branchs, some are living in the cloaca of Holothurians, some in that of Echinids, and others 



again in the tubes of Annelids. The crabs are generally small, the carapace is often ill-calcified, ■', 



membranaceous; regions are not defined; antennulae and antennae very minute; the eye-peduncles \ 



small and very short, slightly movable, the eyes generally present, though often showing signs i 



of degeneration ; the external ma.xillipeds of the typical representatives are peculiarly transformed : \ 



the ischium is indistinguishably fused with the merus and forms with it a broad plate, lying \ 



with its long axis nearly transverse to the long axis of the cephalothorax, and the palp is very \ 

 large; in conformity with this arrangement the buccal cavity is of a semi-lunar shape and very 



broad behind. In other subfamilies, however, the external maxillipeds are lying parallel to each \ 



other with their long axes and of a normal shape, with ischium and merus distinct. \ 



It is on the shape of these maxillipeds that the four subfamilies are founded. 1 



Key to the subfamilies : ' ' 



1. Ischium and merus of ext. maxillipeds distinct 2 



Ischium and merus of ext. maxillipeds fused to a single piece, ; 

 which is usually placed very oblique, almost transverse; palp 



smaller than ischium-merus. Usually the carapace is not appre- ; 



ciably broader than long Subfam. Pinnotherinae - ^ ^ J 



2. Orbits nearly parallel to the longitudinal axis of the body, 



wholly visible in dorsal view Subfam. Xenophthalminae 1 7/ 



Orbits in the usual position, transverse, small 3 ! 



3. Ischium of ext. maxillipeds smaller than merus ; palp often very 



large. Carapace notably broader than long Subfam. Pinnotherelinae - ^ ^ i 



Ischium of ext. maxillipeds larger than merus; palp not large. Subfam. Asthenognathinae :<.7 \jf, 



96 



