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Show 878.] AND DISTRIBUTION OF THE CRAYFISHES. 773 0 (ep r) 2 3 3 4 4 3 + r 1 20 + ep r 4- r. 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 podobranchia? were seen. The first, 0*9 inch long (measured 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 straightened 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 1*05 inch; and it was nearly straight, its apex fitting into the summit of the branchial cavity. The base of each podobranchia 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 podobranchia? 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 remarkably 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 0*35 in. long. Each is concave forwards and convex 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 Somites and their appendages. VII VIII IX X XI XII XIII XIV The Branchial formula of Parastacus PodoArthrobranchia?. branchiae. Anterior. ) (ep r) 0 1 1 1 1 1 1 0 Posterior. 0 0 I 1 1 1 r 0 Pleurobranchiae 0 0 0 0 1 1 1 1 6 4-ep>* + 6 -•- 4 -\-r + 4 = 8. The Branchiee of Astacoides. |
