TRACHEATA. 393 



appearance something like that of many embryo Crustaceans 

 (fig- J 75 B). Between forty and fifty segments are formed while 

 the embryo is still in the egg. The appendages long remain 

 unjointed. The fourth post-oral appendage, which becomes the 

 poison-claw, is early marked out by its greater size : on the 

 third post-oral there is formed a temporary spine to open the 

 egg membrane. 



It does not appear, from Metschnikoff's figures of Geophilus, that any 

 of the anterior segments are without appendages, and it is very probable 

 that Newport is mistaken in supposing that the embryo has a segment with- 

 out appendages behind that with the poison claws, which coalesces with the 

 segment of the latter. It also appears to me rather doubtful whether the 

 third pair of post-oral appendages, i.e. those in front of the poison-claws, can 

 fairly be considered as forming part of the basilar plate. The basilar plate 

 is really the segment of the poison-claws, and may fuse more or less com- 

 pletely with the segment in front and behind it, and the latter is sometimes 

 without a pair of appendages (Lithobius, Scutigera). 



Geophilus, at the time of birth, has a rounded form like that 

 of the Chilognatha. 



The young of Lithobius is born with only six pairs of limbs. 



General observation on the Jiomologies of the appendages of 



Myriapoda. 



The chief difficulty in this connection is the homology of the third pair of 

 post-oral appendages. 



In adult Chilognatha there is present behind the mandibles a four-lobed 

 plate, which is usually regarded as representing two pairs of appendages, 

 viz. the first and second pairs of maxillae of Insects. Metschnikoff's ob- 

 servations seem however to shew that this plate represents but a single 

 pair of appendages, which clearly corresponds with the first pair of maxillae 

 in Insects. The pair of appendages behind this plate is ambulatory, but 

 turned towards the head ; it is in the embryo the foremost of the three 

 functional pairs of legs with which the larva is born. Is it equivalent to 

 the second pair of maxillae of Insects or to the first pair of limbs of Insects? 

 In favour of the former view is the fact (i) that in embryo Insects the 

 second pair of maxillae sometimes resembles the limbs rather than the 

 jaws, so that it might be supposed that in Chilognatha a primitive 

 ambulatory condition of the third pair of appendages has been retained ; 

 (2) that the disappearance of a pair of appendages would have to be 

 postulated if the second alternative is adopted, and that if Insects are 

 descended from forms related to the Myriapods it is surprising to find a 

 pair of appendages always present in the former, absent in the latter. 



