ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 47 



The pleopoda arc very liable to undergo a considerable amount of cor- 

 relation. 



In the ISrachyura the anterior arc adapted for introinittcnt organs, and 

 cany the penis -\vithin their folds. 



In the females of Brachyura and Macrura they arc formed for carrying 

 and suspending the ova in the water, and for swimming purposes in both 

 males and females in tho latter order. 



In the Stomapoda, while they arc used for swimming purposes, in some 

 genera, as Squilla, they support branchial organs also. 



In the Amphipoda they are adapted for swimming only. 



In the Isopoda they are constructed as foliaceous plates, and often enclosed 

 in a chamber and adapted for respiration, though they arc constantly used as 

 natatory appendages also as a secondary condition. This generally refers 

 to the five anterior pairs of appendages. 



The sixth pair universally in Macrura and Isopoda, the fourth, fifth, and 

 sixth in Amphipoda, are variated into leaping or pushing appendages, by 

 winch they are enabled to spring forwards or backwards to a very consider- 

 able extent, as Macrura in the water, and Talitrus and Orchestia on the 

 land. 



The last pair of appendages is seldom present; in one or two genera 

 among the Schizopods they exist in a rudimentary state, but in so feeble a 

 condition that they can only be considered as present in an anatomical sense, 

 as for all functional purposes they are practically useless. 



On Exuviation. 



One of the most curious and interesting phenomena connected with the 

 dermal skeleton is the power of its being cast or shed as a whole. This 

 is exhibited in the Crustacea more perfectly than in any other group of 

 animals. 



The exuviation is not confined to the external or dermal tissue only, but 

 extends to all that which in the higher groups of animals is known as the 

 mucous membrane, or internal continuation of the true skin. 



In all Crustacea, from the smaller Entomostracous forms to the large 

 Podophthalmous animals, every hair and spine upon the external surface, as 

 well as every cilium and minute organ that may exist on the walls of the 

 various internal cavities, is detached and shed in continuous connexion with 

 each other. 



How this could bo accomplished was for a long time a mystery. Accord- 

 ing to Mr. Couch, Olaus AVormeus, in the early part of the seventeenth cen- 

 tury, speaks of it as a thing not to be doubted. The first to observe, with 

 experimental accuracy, the process, was M. Reaumur; as long ago as 1712 

 and 1718 he published, in the ' Me'moires de 1'Academie des Sciences,' pp. 226 

 and 263, his " Observations sur la mue des Ecrevisses." 



He kept specimens of Astaeus Jhtviatilis, or river Crayfish, in cases that he 

 had perforated with holes, and placed in the river stream. He found, in the 

 latter part of the summer or the commencement of the autumn, that these 

 Crustacea change tho skin. He observed that for some days previous to the 

 uommencement of this operation the animal abstains from solid food, 

 from which circumstance ho was able to anticipate tho period of the opera- 

 tion, for he was able by the pressure of his finger on the carapace or surface 



