MOUTH-PARTS OF THE PAL/EMONID PRAWNS. 



39 



For if (as might well be, by bomoeosis) it were independently 

 established in any members of the subphylum, then the altered 

 limbs would lose what they had in common with the corresponding- 

 limbs of other Crustacea. 



3. In point of fact, how T ever, there can be no doubt that the 

 Crustacea are inonophyletic, and it is very highly probable that 

 their ancestor possessed a complete series of similar limbs. The 

 widest gaps in the crustacean system are those which separate 

 the Copepoda and the Cirripedia from the rest of the subphylum, 

 but it is quite impossible even in these cases to entertain the 

 suggestion of an independent origin. The occurrence of the 

 nauplius would by itself negative this. That in the common 

 ancestor of the group all the limbs, with the possible exception « 

 of the antennules, conformed to one type may be gathered with 

 some confidence from the conditions in Branchiopodaand Trilobita. 

 The Branchiopoda are certainly the most primitive of existing 

 Crustacea. They alone possess in a simple form — allowance being- 

 made for certain obvious specializations, such as the degeneration 

 of the mouth-parts — all the elements of the organization of every 

 other class of the subphylum, and their suggestive resemblances 

 to the Annelida are the only indications of affinity with other 

 phyla shown by any recent members of the group. These facts 

 fairly entitle them to be regarded as indicating broadly the 

 ancestral features of the Crustacea. It is needless to dwell upon 

 the fact that their trunk-limbs are all of one type, and, although 

 that type becomes unrecognizable in their maxillae and maxillules, 

 its presence in these limbs of other groups (Ostracoda, Malaco- 

 straca,, and Copepoda) supplies the lacking evidence here. In the 

 Trilobita, which are without doubt related to the forbears of the 

 Branchiopoda, a single type of limb extends throughout the 

 body, from the antennae backwards. It is not possible to avoid 

 attributing to the ancestor of the Crustacea that similarity of 

 appendages which characterizes these groups. This, of course, 

 is not to say that the limbs of the first crustacean resembled 

 closely either those of the Trilobita or those of the Branchiopoda. 

 The form of the primitive crustacean appendage can only be 

 conjectured with probability after comparison of all those which 

 may be assumed to be derived from it. But, whatever it may 

 have been, the foregoing considerations justify the belief that it 

 existed, and that from it can be derived all the post-antennular 

 limbs of every member of the subphylum. 



4. The primitive limb of the Crustacea must have been of the 

 kind to which the terms " leaf-like " and " phyllopod " are 

 applied — that is, flattened, lobed, and feebly, if at all, jointed 

 (text-fig. 2). This is a priori likely in view of the structure of 

 the parapodia of Annelida, and seems established by the way in 

 which the phyllopod limb is distributed among the Crustacea, 

 occurring as it does throughout the trunk of Branchiopoda 

 (text-figs. 3-5), on the hinder and presumably less mature, 

 segments of Triarthrus (text-fig. 6), in Leptostraca (on the thorax, 



