SECT, in THE APPENDAGES 33 



sensory cirrus of the dorsal parapodium of the 

 Annelids. 



The sensory cirrus of the ventral parapodium (r. 7 ) 

 entirely disappears in Apus, but is retained in Limulus 

 (see Fig. 44, p. 192). Both the dorsal and ventral para- 

 podia in Apus carry highly developed setae, as in the 

 Polychaeta, and again, in both, the gill is entirely free 

 from setae, which would hinder the free exchange 

 of the respiratory medium. It may be noted that 

 many Polychaeta have appendages on their parapodia 

 quite as complicated as those on the limbs of Apus 

 known as the endites (B. e.\ whose origin will be 

 discussed later. 



Passing from the Phyllopodan to the typical Crus- 

 tacean limb, we assume that the flabellum is the ex- 

 opodite, the shaft of the limb is the endopodite, and 

 the ventral parapodium is the masticatory ridge. 



I. The first pair of Antenna (Fig. 7 A) This limb 

 has retained its original position at the side of the 

 Annelidan prostomium or upper lip of the Apodidae. 

 It has already been pointed out that the bending of 

 the head has changed its position from behind the 

 mouth to in front of it. 



The form of this limb needs no special comment ; 

 the bend in it is not a true joint, nor is it provided 

 with any muscles except a few which run into the bulb 

 on which it stands. Its setae are modified into sensory 

 hairs, homologous with the olfactory hairs of the 

 higher Crustacea. 



Morphologically, the first antenna must be regarded 



D 



