58 DK. G. C. BOURNE ON THE RANINID^ : 
thorax is effectively prevented. The only entrances and exits are by way of 
the anterior so-called exhalant branchial canals and by the posterior canals 
described above. Bohn (6) has shown that reversal of the respiratory current, 
first observed in Corystes by Garstang, is a normal phenomenon among 
Decapod Crustacea, and is manifested even when they are buried in the sand. 
There must, therefore, be some apparatus in forms which, like the Raninidse, 
have posterior brancliial orifices for filtering the sand from the water entering 
by these orifices during the "normal" phase of the respiratory current; 
tliat is when it is setting from behind forwards. In most of the Raninidse, 
notably in Ranina, Nolopus, Notopoides, NoLosceles, and Zanclifer, this 
apparatus is furnished, partly by the chelipeds, but principally by the manner 
in which the flattened pereiopods, the edges of which are fringed with long 
and closely set hairs, are bent upwards and forwards in such a manner as to 
form a water-chamber on either side of the posterior thoracic segments. 
The part played by the pereiopods would never be guessed from the con- 
ventional representations of these animals in systematic works, where they 
are depicted, usually from the dorsal surface, with the legs extended sym- 
metrically on either side of the body in order to display as much as possible 
of their structure and the chelipeds extended forwards in front of the 
body. The adaptive characters of the thoracic limbs can be studied with 
equal advantage in Notopus, Notopoides, Zandifer, Notosceles, and Ranina, but 
1 will take the last-named genus as an example for descriptive purposes. 
In Ranina there is a conspicuous trianguhir patch of short dense hairs 
extending forward from the articulation of the cheliped over the postero- 
lateral area of the pterygostomial region on either side of the thorax. The 
conjoined basis and ischium of the cheliped is very short and immoveably 
fused to the merus, the two forming a relatively long curved segment of the 
limb, dilated on its external aspect but smooth and flattened internally so as 
to fit closely against the hairy patch on the pterygostome. It is evident, 
from its smooth and polished inner surface, that the ischiomerus is normally 
held close to the body and slides forwards and inwards or outwards and 
backwards over the above-mentioned hairy patch. Whatever its position, so 
long as it is pressed against the hairy patch, there is no room for the passage 
of water, much less of sand or mud, between it and the pterygostome. 
When the ischiomerus is rotated as far forward as possible, the somewhat 
inflated carpus lies beneath the outermost of the large frontal spines of 
the carapace, and the flattened propodus and dactylus are folded back 
under the anterior part of the pterygostomial region, but do not fit closely 
and accurately to the latter as in the case of Calappa and Matuta. Their 
function is quite different. It is evident that instead of forming the floor of 
what Garstang has called an exostegal canal the propodus is so articulated 
to the carpus that without either the latter or the ischiomerus being shifted 
from their [lositions, it can be rotated outwards in such a manner as to rake 
