TR AC HE AT A. 325 



formed successively from the posterior unsegmented part. The 

 antennas are the first appendages to appear, and are well developed 

 when eighteen segments have become visible (fig. 175 A). The 

 post-oral appendages are formed slightly later, and in order from 

 before backwards. As the embryo grows in length, and fresh seg- 

 ments continue to be formed, the posterior part of it becomes bent 

 over so as to face the ventral surface of the anterior, and it acquires an 

 appearance something like that of many embryo Crustaceans (fig. 175 

 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 MetsclmikofFs 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 some- 

 times 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 Pomologies 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 maxilla? 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 niaxillse 

 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 maxilla? of Insects or to the first pair of limbs of Insects] 

 In favour of the former view is the fact (1) that in embryo Insects the 

 second pair of maxillse 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. 

 The arguments which can be urged for the opposite view do not appear 



