OCR Text |
Show 1878.] AND DISTRIBUTION OF THE CRAYFISHES. "81 The Branchial formula of Peneeus brasiliensis. Somites and their appendages VII.. VIII.. IX.. X.. XL. XII. XIII.. XIV.. Podobranchia?. 0(ep) 0(ep) 0(ep) 0(ep) 0(ep) 0(ep) 0 0 Arthrobranchia?. Anterior. Posterior. I (small) 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 Pleurobranchia?. 0 = 1+ep 3 + ep 3 + ep 3 + ep 3 + ep 3 + ep 3 1 0 + 6ep+7 + 6 + 7 = 20 + 6 ep. The arthrobranchia of VII. is very small; in fact, it is a tuft of dichotomously divided branchial filaments. The results of this examination of the principal forms of those Trichobranchiata which possess more than one kind of branchiae mav be thrown into a tabular form, as follows:- I. No appendages to the first abdominal somite in either sex. Astyla. 1. Podobranchia? undivided..., PARASTACID.E. 2 Podobranchia? divided into branchial and epipoditic portions PALINURIDJE. II. Appendages of the first abdominal somite almost always present, and specially modified in the males. Stylophora, 1. Podobranchia? partially divided POTAMOBIIDAE. 2. Podobranchia? completely divided or reduced to epi-podites. a. Pleurobranchia? not more than four. a. Posterior pleurobranchia present, and the most posterior of the podobranchia? complete HOMARID.E. Homarus, Nephrops. j8. Posterior pleurobranchia wanting, and more or fewer of the posterior podobranchia? reduced to epipodites. a. Branchia? with only filamentous processes Axius. b. Branchia? with foliaceous as well as filamentous processes Thalassina. b. Pleurobranchia? more than four. a. Branchia? filamentous Stenopus. (3. Branchia? ramose Peneeus. All the Trichobranchiata are Macrura, in the sense of having abdomen and its appendages well developed; but, in Ibacus and Thenus, the abdomen becomes shortened and the cephalothorax wide, while the antennary and the ocular regions are modified in a manner very similar to that which gives rise to the peculiar "face" or " metope " of the typical Brachyura. A very little further modification would convert Thenus, for example, into a trichobranchiate Crab. Such forms as these, which simulate the Brachyura, and yet differ profoundly from them, may be termed " pseudo-carcinoids." |