CRUSTACEA OUTER ORGANISATION 

 f. The Parag-natha. 



315 



We may here in passing notice certain peculiar processes which in 

 the Thoracostraca and a few Entomostraca (Ostracoda and Copepoda) rise 

 independently between the mandibles and maxillae on the ventral 

 integument of the head, and are called paragnatha. They cannot be 

 considered as separate limbs, as they are never innervated by special 

 ganglia. They may perhaps represent the proximal masticatory ridges 

 of the anterior maxillse which have become independent. In Apseudes 

 (Anisopoda) a ridge-like portion is marked off' from them. 



2. The Limbs of the Trunk. 



The limbs of the trunk can be deduced from biramose feet. Their 

 number varies in the Entomostraca, but is constant in the Mcdacostraca. 

 In the latter we always distinguish, in correspondence with the 

 segmentation of the trunk, 8 pairs of thoracic limbs and 6 pairs of 

 abdominal limbs or pleopoda. 



There is nothing to prevent us from assuming that the trunk 

 appendages of the Entomostraca correspond with those of the Mala- 

 costraca, pair with pair from before backwards. 



a. Entomostraea (Fig. 215). 



Phyllopoda. The Branchiopoda, and Cladocera must be described separately. 

 Branchiopoda (Fig. 214 ; Fig. 215, C) : the numerous (10 to 36) pairs of trunk limbs are 

 formed pretty much alike. They are 

 wanting on a varying number of pos- 

 terior (abdominal) segments. All the 

 limbs are leaf - shaped swimming feet 

 with branchial appendages. They also 

 serve for whirling food within reach. 

 Their structure is essentially as follows. 

 An unjointed or indistinctly jointed stem 

 carries on its inner side (that turned to 

 the median plane of the body) 6 appen- 

 dages or lobes (endites) and on the outer 

 side a flat respiratory plate and a pouch- or 

 sac-like branchial appendage (epipodite). 

 It is at present impossible, without 

 straining, to deduce all these parts from 



FIG. 214. Apus. Transverse section in the 

 neighbourhood of the 7th or' 8th pair of feet. 

 h, Heart ; d, intestine ; ov, ovaries ; 5m, ventral 

 chord ; ah, respiratory cavity between the shell 

 (s) and the body ; 1-6, endites ; 6r, gills ; ex, respir- 

 there are 10 to 2/ pairs of swimming feet. atory plate (after Packard). 



The respiratory plate is divided into two. 



The Apodidcc (Apus) usually possess 35-50 pairs of swimming feet. The endites 

 are jointed, and may be described as flagellate appendages ; they are very long (the 

 5th especially) on the 1st pair of swimming feet. The llth pair of feet carry on 



the typical component parts of a biramose 

 limb. The respiratory plate is generally 

 held to be an exopodite. In the Limna- 

 diadce (Limnetis, Estheria, Limnadia) 



