GENERAL CONSIDERATIONS. 315 



as yet unlengtliened form of Pliyllopodan limb, which recurs in Ncbalia and in 

 the maxillae of the Copepoda and Malacostraca, approaches the primitive type, 

 and that the elongate forms of limb have been secondarily developed from this. 

 We shall then be able to derive the biramose character of the Crustacean linil) 

 direct from the correspondingly shaped Annelidan parapodium, and should 

 perhaps be justified in deriving the epipodial appendages of the Crustacean 

 limb from the dorsal gills of the Annelidan parapodia. On the other hand, 

 the greater mobility of the segmental appendages led to the development of a 

 new function, viz., the interaction of tlie two limlis of one and the same pair, 

 this again leading to the development of corresponding loliate structures 

 (endites, masticatory ])rocesses) on the mesial sides of the limbs. Sucli 

 processes occur on all the trunk-limbs of the Branch iojsoda, and are there 

 used for the j)urpose of passing on particles of food. In most Crustacea, on 

 the contrary, the accessory structures are limited to the limbs surrounding the 

 mouth. It is a fact worthy of special attention that the second antenna in 

 the Nauplius stage also participates in the w'ork of mastication by means of the 

 masticatory process on its basal joint, and attains its pre-oral position only at a 

 later stage, when it loses its masticatory function. 



If we imagine a Protostracan serving as a racial form of the 

 Crustacea produced out of an Annelidan type by the modification 

 above described in the method of locomotion, and the changes in 

 the limbs (with which is connected a modification in the condition 

 of the body-cavity), it becomes evident that we cannot attribute to 

 it all those characters by which the group of the Crustacea is distin- 

 guished. The union of the five anterior limb-bearing segments to 

 form a common region of the body (head), the transformation of 

 the two anterior pairs of limbs into typical Crustacean antennae, 

 the development through the Nauplius — these are characters which 

 occur in all Crustacea, and which we shall also attribute to the 

 primitive Fhi/l/opod, but which need not utcessarily l)e claimed for 

 the Protostracan form. We may, on the contrary, claim a great 

 latitude of variation for this latter. We shall have to assume that 

 the hypothetical Protostracan group comprised forms of life far 

 removed from the typical Crustacean structure. As such stocks 

 produced independently from the Protostracan group, we may point 

 to the Palaeostraca (Trilobita, Gigantostraca, Xiphosura), as also to 

 the Pantopoda, of which we shall treat later. 



It should here be pointed out that Peripatics also, in a few respects, 

 shows wonderful agreement with the Crustacea. The researches 

 of Sedgwick have revealed a great similarity in the structure of 

 the nephridia in the two groups. The relation of the maxillary 

 ganglion to the brain in Peripatus recalls the corresponding relation 

 of the antennal ganglion in the Crustacea. It has further become 

 probable that a structure in Peripjaius, until recently little observed, 



