THE LARVAL DEVELOPMENT. 135 



a transitionary stage, in which the genital aperture is already shifted 

 further back than in Fig. 60. The differentiation of a vagina from 

 the primitive genital duct has here already taken place. In Fig. 62, 

 the genital aperture has already assumed its final position near the 

 anus. 



Stiles speaks of a differentiation of the sexes at an early stage ; but the 

 stages described by him in P. proboscideum seem to us to be somewhat more 

 advanced than those observed by Leuckart in P. taenioides. 



According to Hoyle, it appears that the genital glands may originally have 

 been paired. If this were the case, we should have, in the fusion of the germ- 

 glands to form a single organ, a process similar to that in the Acarina (p. 101). 

 The position of the (female) genital aperture at the posterior end of the body, 

 which is in opposition to what is usual in the Arachnida, might, according to 

 Leuckart's explanation, be regarded as secondary. 



The body of the encysted larva after the first moults looks quite 

 smooth, but later a series of rings make their appearance (Fig. 62). 

 These first arise in the middle of the body, and spread anteriorly 

 and posteriorly. These superficial markings cannot be regarded as 

 equivalent to actual segmentation on account of their late appearance 

 and their development. In some Pentastomidae, e.g., P. protelis 

 (Hoyle), they are somewhat broad, and constrictions form between 

 them, thus increasing the resemblance to a true segmentation. In 

 P. proboscideum also such an appearance can be remarked, and is still 

 more conspicuous in P. taenioides. In other Pentastomidae, raised 

 rings are found like broad hoops round a barrel, separated by inter- 

 spaces (Van Beneden, Jacquart). 



Small circular apertures in the chitinous integument are found 

 distributed all over the surface of the body, and later, in consequence 

 of the formation of rings, arranged in transverse rows upon it. These 

 resemble the two glandular stigmata of the four-limbed larva 

 (Fig. 62), and were regarded by Leuckart as the apertures of 

 integumental glands. A differentiation of the chitinous covering of 

 the body which arises in later larval stages is found in the so-called 

 circles of spines which appear at the posterior edge of each ring, and 

 are characteristic of the fully-formed larva (Fig. 63, st). The larva 

 of P. taenioides, which was formerly taken for a sexually mature 

 form and called P. denticulatum, has the circles of spines specially 

 well developed. They are probably of advantage to the animal in 

 locomotion. Still more important aids in locomotion and attachment 

 are the hooks — two pairs of claw-like chitinous structures (Fig. 63, 

 h), which develop in two sac-like depressions of the integument in 

 front of the mouth (Fig. 62, Jit). The hooks have no connection 



