558 E. RAY LANKESTER. 



the typical "biramose limb" often found in Crustacea. The 

 rami may be flattened for swimming, when it is " a biramose 

 swimmeret/' or both or only one may be filiform and finely 

 annulate; this is the form often presented by the antennae 

 of Crustacea, and rarely by prasoral appendages in other 

 Arthropods. 



(7) The endopoditic ramus is greatly enlarged and flat- 

 tened, without or with only one jointing, the corm (basal 

 segment) is evanescent; often the plate-like endopodites of a 

 pair of such appendages unite in the middle line with one 

 another or by the intermediary of a sternal upgrowth and 

 form a single broad plate. (These are the plate-like swim- 

 merets and opercula of Gigantostraca and Limulus among 

 Arachnids and of Isopod crustaceans. They may have rudi- 

 mentary exopodites, and may or may not have branchial 

 filaments or lamellee developed on their posterior faces. The 

 simplest form to which they may be reduced is seen in the 

 genital operculum of the scorpion.) 



(8) The gnathobase becomes greatly enlarged and not 

 separated by a joint from the corm ; it acts as a hemignath 

 or half-jaw working against its fellow of the opposite side. 

 The endopodite may be retained as a small segmented palp 

 at the side of the gnathobase or disappear (mandible of 

 Crustacea, Chilopoda, and Hexapoda), 



(9) The corm becomes the seat of a development of a 

 special visual organ, the Arthropod eye (as opposed to 

 the Chcetopod eye). Its jointing (segmentation) may be 

 retained, but its rami disappear (podophthalmous Crustacea). 

 Usually it becomes atrophied, leaving the eye as a sessile 

 organ upon the prteoral region of the body. (The eye-stalk 

 and sessile lateral eyes of Arthropoda generally, exclusive of 

 Peripatus.) 



(10) The forms assumed by special modification of the 

 elements of the parapodiiim in the maxillae, labium, etc., 

 of Hexapods, Chilopods, Diplopods, and of various Crustacea 

 deserve special enumeration, but cannot be dealt with with- 

 out ample space and illustration. 



