18/8.] AND DISTRIBUTION OF THE CRAYFISHES. 783 



of De Haan), if we exclude the Peneeidce, constitute a natural assem- 

 blage, to which I will apply the name of " Caridomorpha." They are 

 all eminently Macrurous ; and the characteristic feature of their 

 branchial system is the predominence of the pleurobranchiee, and the 

 concomitant diminution in the number and the importance of the 

 arthrobranchiae and of the podobranchia?. In fact, so far as I am 

 aware, there are never any traces of the latter except upon the 

 maxillipedes. 



In both Palcemon and Crangon I find five pleurobranchiae attached 

 to the posterior thoracic somites, from the tenth to the fourteenth 

 inclusively. In Palcemon, two arthrobranchiae, one of which is very 

 small, are attached to the arthrodial membrane of the external maxil- 

 lipede, which has a very short and rudimentary epipodite. The second 

 maxillipede bears a podobranchia divided into a small branchia and 

 an oval epipoditic plate. In the first maxillipede the place of the 

 podobranchia is occupied by a rounded bilobed lamella. 



In Crangon none of the maxillipedes bear gills. The epipodite 

 of the first maxillipede is relatively much larger and triangular ; 

 that of the second is tongue- shaped and almost vesicular ; that of 

 the third is a mere rounded process. 



I can find only one arthrobranchia on the ninth somite. 



The Branchial formula q/Talsemon. 



Somitea „ , Arthrobranchia?. t,, 



_ j .-, ■ Podo- Pleuro- 



and their , , . ^ — * ^ . , . 



appendages. brallchl *- Anterior. Posterior, branehi*. 



VII 0(ep) = 0(ep) 



VIII ] 0=1 



IX (ep) 1 1 = 2 + ep 



X 1 = 1 



XI 1 = 1 



XII 1 = 1 



XIII 1 = 1 



XIV 1 = 1 



H-2ep+l + 1 + 5 = 8 + 2 ep. 



From the number of their pleurobranchiae the Caridomorpha can- 

 not be regarded as a reduced modification of any of the Tricho- 

 branchiata, except the Penaeidae and Stenopus. But it is easy to 

 derive them from a Stenopus-like primitive form by the reduction 

 of the podobranchise and the arthrobranchiae, and the conversion of 

 the five posterior pleurobranchiae into gills of the lamellar type. 



In the Brachyura of Milne-Edwards the disposition of the bran- 

 chial apparatus is well known to be definite and characteristic. In 

 Cancer pagurus, for example, there are nine branchiae ; seven of these 

 are pyramidal in form, and take a general direction from the base 

 towards the apex of the branchial chamber, to the inner walls of 

 which they are applied. The two posterior of these gills are pleuro- 

 branchiae, being attached respectively to the epimera of the eleventh 

 and the twelfth somites. The fifth and fourth, the third and second, 



