REPORT ON THE AMPHIPODA. xiii 



the feebler form as the feebler have been degraded or reduced from the more ample, 

 the answer is this, that the impoverished condition attributed to the pleon in the 

 Caprellina is correlated to other appearances of degradation in the same group, that 

 no Caprelline stage has ever been observed in the embryos of the other two groups, 

 and that the strongly developed pleon would scarcely have been so general, not only 

 among Amphipods, but also in the neighbouring orders of Crustacea, had the ancestral 

 form been nearest to the rudimentary one. Hence it appears to be a natural arrange- 

 ment to place the Caprellina after, though next to, the Gammarina, and to let the 

 Hyperina come last. 



To any one glancing over the great variety of forms presented by different species 

 of Amphipoda, and comparing an Orchestia with a Cystisoma, a Rhabdosoma with an 

 Anonyx, a Pariamhus with a Gammaracanthus, it might seem extremely rash to assume 

 that all the Amphipoda could possibly have a lineage in common. But after prolonged 

 examination of homologous parts the observer would not be so much impressed with 

 the difficulty of a common descent as with the intrinsic simplicity of the processes by 

 which these wonderful diiferences of structure might have been produced. For if a son 

 may be taller than his father, a daughter stouter than her mother, in the same family one 

 child have straight hair and another curls, one brother be smooth and the other a hairy 

 man, variations of a corresponding kind suffice to explain the most strikiug dis- 

 similarities that the Amphipoda can furnish. Lengthen or contract a limb, make a joint 

 tumid or flatten it out, multiply the spines or prickles, narrow or expand the body, or 

 so treat one part of it at the expense of another, let it be cylindrical or depressed or 

 laterally pinched, stiffly outstretched or coiled into a ball, — by such ditferences as these, 

 in regard to which many species present the most minute transitions, it will be found 

 that genera and families are separated, without the least necessity or reasonableness of 

 attributing to them other than a common origin. 



In the hinder part of the pleon the Hyperina show a general but very variable agree- 

 ment with the Gammarina, but in the front part of the pleon, and especially in the 

 appendages of that part, the agreement is great and very constant. These append- 

 ages, commonly called pleopods, are perhaps less subject to variation throughout the two 

 groups than any other part of the organism. Each of the first three segments of the pleon 

 has a pair of these swimming-feet, the three pairs usually differing only a little one from 

 the other ; each member of a pair consists of a stem or peduncle supporting two branches ; 

 the branches as a rule differ only slightly from one another, each being of tapering form, 

 composed of several joints, of which the first is invariably the largest ; of these joints 

 every one has an apical pair of long feathered setse, which on the small terminal 

 joint are close together. No joint except the first is ever privileged to have more than 

 one pair of these plumes, and no joint is ever normally without its pair. 



On the peduncles of these swimming-feet, near the lower angle on the inner side, there 



