352 



PARASITISM 



tion functions as a respiratory pore (" soupirail " of Pantel) and 

 the parasite is maintained in the body-cavity of the host, with its 

 spiracular extremity firmly wedged in the aperture. The sheath 

 consists of the same layers as the integument, its lining being 

 continuous with the external cuticle. It forms a stout funnel, 

 which grows around the hindmost segments of the parasite, and 

 its walls often become thickened by the incorporation of fat cells, 

 dead phagocytes and other tissues which become bound together 

 by a copious infiltration of chitin produced by the hypodermal 

 layer. Distally the sheath is membranous, and may entirely 



enclose the larva and, under 

 such circumstances, it 

 appears to be permeable to 

 the body-fluids of the host 

 which form the sole 

 nutriment of the parasite 

 within. The first two 

 ecdyses commonly occur 

 while the parasite is thus 

 enclosed, and the exuviae 

 cast off within the 

 of the sheath. 

 Ultimately the parasite 

 either frees itself completely 

 from this investment and 

 becomes actively carni- 

 vorous among the organs of the host ; or, it ruptures the 

 membranous bag, but continues to respire the outer air with its 

 spiracular extremity still retained within the basal funnel of the 

 sheath (Fig. 88). 



A secondary sheath may be either cutaneous or tracheal in 

 origin (Fig. 89), and is formed with respect to species whose early 

 life is passed either free in the body-cavity of the host, or within 

 an individual organ of its body, usually a nerve centre or the 

 mid-intestine. It is formed at a stage when the respiratory needs 

 of the growing parasite demand a more adequate supply of oxygen 

 than can be derived from the body-fluids. The parasite fulfils its 



are 



Fig. 88. Section through a larva of cavitv 

 Thrixion attached to cutaneous respira- 

 tory funnel /. a. Primary entrance hole 

 through integument e of host. sh, 

 Integumental sheath enclosing parasite. 

 s, Spiracle. (Adapted from Pantel.) 



