1878.] AND DISTRIBUTION OF THE CRAYFISHES. 773 



The Branchial formula of Parastacus. 



Somites -r, , Arfchrobranchiae. t,, 



■, ,, . Podo- Pleuro- 



and their , , . ,. * v , , . 



, branchiae. . , „ , j. branchi.T. 



appendages. An tenor. Posterior. 



VII O(epr) = (ep r) 



VIII I 1 0=2 



IX 1 1 I 0=3 



X 1 1 I 0=3 



X! 1 1 1 1=4 



XII 1 1 1 1=4 



X1H 1 1 r 1 = 3 + r 



XIV 1=1 



6 +ep>'+ 6 -j- 4 -fr+4 = 20 + ep ;• + r. 



8. The Branchiae of Astacoides. 



I am indebted to the courtesy of Prof. Alphonse Milne-Edwards 

 for the opportunity of examining the branchiae of a male specimen 

 of the Crayfish of Madagascar, Astacoides madagascariensis. On 

 account of the rarity of this species, it is desirable to describe its 

 branchial apparatus in some detail. The length of the specimen 

 was 5 - 7 inches. 



The branchiostegite of the left side being carefully removed, the 

 six large podobranchiae were seen. The first, 0"9 inch long (mea- 

 sured from its attachment to its apex), was directed upwards and 

 backwards in the cervical depression. The second, 1 inch long, 

 took a similar direction, but was concave forwards. The third, 

 slightly shorter, lay parallel with the second. The fourth (fig. 7, I, 

 n) was much longer ; bent round the third, its summit touched the 

 apex of the second ; the total length of the branchia when straight- 

 ened out was about 1 - 3 inch. The fifth branchia was still longer, 

 curving round the posterior edge of the last, so that its apex touched 

 the front boundary of the branchial cavity. The sixth branchia 

 started from a higher level than any of the others, in consequence of 

 the great size of the coxopodite of the penultimate thoracic limb ; its- 

 length was l - 05 inch; and it was nearly straight, its apex fitting 

 into the summit of the branchial cavity. The base of each podo- 

 branchia is elongated upwards, as in Astacopsis ; and there is a soft 

 fold of integument over its attachment. There is no trace of any 

 lamina. 



On the removal of the podobranchiae the arthrobranchiae came 

 into view. But, in contradistinction to all other known Crayfishes, 

 there are only five of them fully developed, and even these are re- 

 markably small in comparison with the podobranchiae. The first 

 is the shortest (0 - 5 in.) ; and they increase in length to the hinder- 

 most, which is 035 in. long. Each is concave forwards and con- 

 vex backwards; and the apex of the hindermost comes within 0*15 

 inch of the nearest part of the anterior superior boundary of the 

 branchial cavity. In all these branchiae the branchial filaments 

 are very numerous, stiff, relatively short and close-set ; so that they 



Proc. Zool. Soc— 1878, No. LI. 51 



