Bespiratory Current in the Decapods. 23 



in Carcinus mcBnas, in whicli the reversal is pronounced, nor 

 in Portumnus latipes and Porcellana longicornis ; but it is 

 found in Portunus arcuatus and Pilumnus hirtellus^ which 

 belong to categories III. and II. of the table. Gallianassa, 

 which is included in the latter category, is infested with 

 lonidaB allied to Cepon. 



Bopyrus, properly so called, is partial to the species of 

 Palcemon, Hippolyte, and Galatkea, belonging to category II. 



Copepods are rare : they have been met with only in a 

 few examples of Hippolyte and in the lobster. Excludnig 

 this latter and a few rare exceptions *, the extreme categories 

 of the table comprise only species which are without branchial 

 parasites. 



I have attempted to account for these facts, and it seems 

 to me that the reversal alluded to has both disadvantages and 

 advantages for the parasite. 



An inverse current can always eject the parasite in the act 

 of penetrating into the branchial cavity, just as it also expels 

 any strange body whatever. The Entoniscidee alone are able 

 to penetrate in the case of the species in which the reversal 

 is somewhat prolonged ; Cepon and Bopyrus appear much 

 more susceptible in this respect. 



But an inverse current, while it cleanses the branchial 

 chamber, causes to pass through it a much more highly 

 oxygenated stream of water, for it is more rapid and the 

 water has not time to charge itself (especially from the well- 

 developed posterior gills) with carbonic anhydride. Here 

 there is an advantage for the parasite ; but in the case of the 

 Entoniscida3, in which the respiratory mechanism has been 

 discovered by M. Giard, the advantage is more apparent. 

 In the midst of the animal's viscera the parasite occupies a 

 chamber which is in communication with the branchial cavity 

 of the host by means of a narrow orifice ; in consequence of 

 the movements of the abdominal appendages of the parasite 

 the water is drawn in and driven out again alternately. 

 Now in the branchial cavity of the host, which from time to 

 time reverses the circulation of the water, there is a similar 

 alternation : to the inverse current corresponds an increase of 

 pressure of the water in this cavity, which forces the liquid 

 to penetrate into the chamber of the parasite ; to the direct 

 current tliere corresponds, on the contrary, a diminution of 

 pressure, which assists the parasite's expiration. 



* Pcu/urus Bernhardus is infested with a Palceogyye, but it is true the 

 latter is very rare. 



