TYPICAL FORM OF THE CRUSTACEAN LIMBS. 



195 



hooks, lamellae, or lobes, or else as masticatory blades. We have 

 already observed the development of such a masticatory hook on 

 the protopodite of the second antenna (Fig. 89, p. 191). 



Another component of the Crustacean limb which, however, is 

 not always developed, is to be found on the external side of the 

 protopodite, and is in most cases connected with the movement of 

 the surrounding water and has a secondary respiratory function. 

 Such appendages, several of which may occur together, we shall 

 call epipodites (Fig. 91, ep), whether they are developed as lamellae 

 (epipodial plate, ep) or as branchial sacs or tubes (ep 1 ) provided with 

 a rich circulation of blood (p. 172). The epipodial appendages, as a 

 rule, belong to the coxopodite or basal joint. 



In the following pages we shall have to refer to the many modifiations and 

 degenerations which these separate constituents of the Crustacean limb undergo. 

 Of the two extreme types, the elongate form of limb appears to be the more 

 specialised and to have developed very gradually, while the lamellate, broadened 

 form, such as occurs in most Phyllopodan limbs 

 and in the thoracic limbs of Nebalia, probably 

 represents the more primitive type. 



It has not yet been satisfactorily demonstrated 

 in what manner the separate parts of the leaf- 

 like Phyllopodan limb can be homologised with 

 those of the schematic Crustacean limb above 

 described. Ray Lankester's explanation seems 

 to have a certain degree of probability. Accord- 

 ing to this author, among the six endites belong- 

 ing to the inner side, the fifth represents the 

 endopodite (Fig. 92, en 5 ), and the sixth the 

 exopodite (en 6 ). This view is supported by 

 the observations of Claus (Nos. 20 and 21), 

 which show that, in the developing limb of 

 Branchipus, these two lobes were the first to 

 become distinct. The fan-like plate (ep), which 

 arises much later, is to be regarded as an epipodial 

 appendage as well as the branchial sac (ep 1 ). 

 The breaking up of the stem of the limb into 

 separate joints shown in Fig. 92 is exceptional 

 among the Phyllopoda. 



We feel tempted to deduce the typical biramose 

 form of the Crustacean limb direct from the 

 bilobed Annelidan parapodium. Such a deduction 

 is supported by the fact just mentioned that the 

 separation of the exopodite and endopodite takes 

 place unusually early in the limb-rudiment in Branchipus (Claus). Whilst, 

 however, the Annelidan parapodia are processes of the lateral sides of the body, 

 the limbs of the Crustacea have shifted ventrally and have become approximated, 

 a fact which indicates that we must imagine, as the intermediate form between 

 the Annelida and the Crustacea, an animal which crept on the sea-bottom. The 



Fig. 92.— Second thoracic limb of 

 Apus cancriformis (after Ray 

 Lankester). 1, first, 2, second 

 stem-joint; eni, en 2 , en^, en 4 , 

 en 5 , en$, the six endites ; ep, 

 epipodial lamella; ep', bran- 

 chial sac. 



