ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 93 



the greater. Sometimes the male appendage is developed so monstroiisly 

 that they appear inconvenient and burdensome, and are occasionally so long 

 that they arc useless in an attempt to reach the mouth. Thus in Homarus 

 the animal feeds itself with the small posterior pair. In GeJassirmis no 

 ingenuity on the part of the animal would enable it to reach the mouth with 

 the extremities of the large chelate organ. In the process of feeding they 

 are useful only as holding food while the animal carries it to the mouth with 

 the smaller but more convenient organs. The chela is always formed by the 

 greater or less amountof development that is given to the inferior angle of the 

 distal extremity of the antepenultimate joint. This power of production ap- 

 pears to be dormant in every limb, since we see it occasionally exhibited in all. 

 Thus in Palinurus it is rudimentarily present in the posterior pair of pereio- 

 poda, and in the genus Pa<jnrus it is developed into a small but efficient organ, 

 by which the animal cleanses out and removes obstructive objects that may 

 have found their way into the branchial chamber, and so fulfils the same 

 duties as those performed by the fiahella attached to the gnathopoda, and 

 which are wanting in the Anomura. 



The fact that the coxtc of all the legs attached to the pereion are in some 

 orders absorbed into the sternal plastron, while they are not so in others, 

 offers a ready and safe means by which pahieontologists may determine the 

 order to which a fossil Crustacean might belong by the evidence of a single 

 leg. Thus it will be seen invariably that seven distinct and free joints are 

 visible in the Macrura, while only six are free in the Brachyura ; whereas 

 in the Anomura there are six free and one partially so. This evidence 

 might be carried still further, inasmuch as in Astacus and Homanis the coxae 

 are seen to approximate to each other on the opposite sides closely, while 

 in Palinurus thej- are near anteriorly and broadly separated posteriorly. 



The appendages that follow are those that are modified for swimming. 

 When exhibited in the most normal condition, they consist of a long pedun- 

 cular stalk supporting two oblong leaf-like plates, surroiiuded by a fringe of 

 small hairs. Sometimes they consist of a scries of multiarticulations, as in 

 Amphipoda ; sometimes of long cylindiical imiarticular branches, as in Cancer. 

 In some instances, as in tSquUla, there is a third branch that springs from 

 the side of the peduncle near the base ; this is so membranoiis in character 

 and ramified in construction, that it is evidently formed for the purpose of 

 assisting in aeration of the blood. 



The pleopoda are utilized, according to the habits of the animal, for various 

 purposes, and throughout them all their adaptation to propulsion through the 

 water is not only the most constant but also very generally associated with 

 other offices. 



In the Isopoda they appear to bo the only organs adapted for respiration 

 that the animal possesses. Tet their rapid motion is the only means which 

 the)' possess of swimming. 



In the Amphipoda, it is this latter use alone for which these organs are 

 adapted, while respiration is fulfilled by other means. But here only the ante- 

 rior three pairs are adapted for swimming purposes, while the posterior three 

 are utilized for leaping when on laud, or forcibly dashing through the watei-. 

 The Isopoda have only the posterior pair so variated, and the Macrura have two 

 pairs; but in this latter order they are more adapted for producing a retro- 

 grade motion, darting backwards as they frequently do to a^oid unexpected 

 and sudden danger. In the Macrurous forms they are also available for the 

 purpose of retaining connexion with the ova, and supporting the life of the 

 embryo until it is matured. Throughout most of the Macrurous forms the 

 pleopoda fulfil this doiible purpose in the female. 



