'860 



CLASS V. ARACHNIDA. 



II 



D 



the egg-shell disappears and the issuing larva makes its way into 

 the tissues surrounding the alimentary canal. Here it comes 



to rest and encysts, until this first host 

 falls a prey to some carnivorous animal, 

 when the parasite makes its way into the 

 nose or lungs and becomes in a short 

 time mature. 



Like most Entozoa and other animals 

 that live in the dark, the members of 

 the Pentastomida are white unless tinged 

 with the blood of their host which they 

 suck. The body is either cylindrical or 

 in the genus Linguatida somewhat flat- 

 tened and lanceolate ; it is more or less 

 sharply ringed, but the annuli do not cor- 

 respond with any internal segmentation 

 and there is very little trace of segmenta- 

 tion in the body at any time of the life- 

 history. 



The anterior end of the body is separ- 

 ated off from the rest sometimes by quite 

 a narrow neck. This head bears the 

 mouth and 'two pairs of chitinous hooks. 

 It never shows any annulations. The 

 hooks have expanded chitinous basal 

 plates into which muscles are inserted. 

 The hooks themselves are strongly re- 

 curved and can be protruded and retracted with ease. 



Certain papillae are symmetrically scattered over the head, 

 and others are arranged over the anterior half of each annulus. 

 These correspond with epidermal pits. There is also a pair 

 of large papillae, which lie in front of the inner pair of hooks 

 and bear the openings of the hook-glands. 



The body is ensheathed in a cuticle which is secreted by the 

 epidermal cells. The epidermal glands are special aggregations 

 of these cells with a short lumen opening on the papillae : these 

 openings were formerly called stigmata. Some authorities regard 

 them as having an excretory function.* The hook-glands how- 

 ever are much larger structures, and extend on each side of the 

 * Baldwin Spencer, Q. J. Mic. Sci., xxxiv. 1893, p. -1. 



Fio. 569. Pentaslonium 

 tvenimdes. O Mouth ; 

 Hf the hooks ; D m- 



. testine ; A anus. 



