T RAG HE AT A. 



323 



iiected with it by a structureless membrane which is probably the outer of 

 the r,wo cuticular membranes. 



At the time when the embryo of Strongylosoma is hatched (fig. 

 174 B) nine post-cephalic segments appear to be present. Of these 

 segments the second is apparently (from Metschnikoff's figure, 174 B) 

 without a pair of appendages; the third and fourth are each provided 



A B 



FIG. 174. Two STAGES IN THE DEVELOPMENT OF STRONGYLOSOMA GUERINIT. 



(After Metschnikoff.) 



A. A seventeen days' embryo, already segmented. 



B. A just-batched larva. 



with a single functional pair of limbs ; the fifth segment is provided 

 with two pairs of rudimentary limbs, which are involuted in a single 

 sack and not visible without preparation, and therefore not shewn 

 in the figure. The sixth segment is provided with but a single pair 

 of appendages, though a second pair is subsequently developed on it 1 . 

 Julus, at the time it leaves the chorioii, is imperfectly segmented, but is 

 provided with antennae, mandibles, and maxillje, and seven pairs of limbs, 

 of which the first three are much more developed than the remainder. 

 Segmentation soon makes its appearance, and the head becomes distinct 

 from the trunk, and on each of the three anterior trunk segments a single 

 pair of limbs is very conspicuous (Metschnikoff) 2 . Each of the succeeding 

 segments bears eventually two pairs of appendages. At the time when 

 the inner embryonic cuticle is cast off, the larva appears to be hexapodous, 



1 Tbougb the superficially hexapodous larva of Strongylosoma and other Cbilo- 

 gnatba has a striking resemblance to some larval Insects, no real comparison is pos- 

 sible between them, even on the assumption that the three functional appendages of 

 both are homologous, because Embryology clearly proves that the hexapodous Insect 

 type has originated from an ancestor with numerous appendages by the atrophy of those 

 appendages, and not from an hexapodous larval form prior to the development of the 

 full number of adult appendages. 



2 Newport states however that a pair of limbs is present on the first, second, and 

 fourth post-oral segments, but that the third segment is apodous; and this is un- 

 doubtedly the case in the adult. 



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