136 



PENTASTOMIDAE. 



with the truncated limbs of the larva, nor can they be regarded as 

 limbs, as might appear from their origin as depressions and in front 

 of the mouth. At a later stage they shift further back towards, or 

 even behind, the mouth (Fig. 63). A further differentiation of the 

 surface is found in the appearance of a large number of papillae 

 arranged in pairs at the anterior end of the body (Fig. 63, tp), which 

 have been considered tactile organs (Leuckart, Stiles). 



The last larval form and its transference to the final host. 

 While these external and internal ontogenetic 

 processes have been taking place the body 

 has lengthened, and has thus been forced 

 to curl up in the cyst, within which 

 the general form of the adult animal is 

 reached. The larva (Fig. 63) now breaks 

 through the cyst, and wanders away from the 

 part hitherto inhabited by it, the circles of 

 spines assisting it in this process. Should the 

 intermediate host in which it lives at this time 

 fall a victim to a beast of prey, the larva possibly 

 passes direct out of the mouth of the latter into 

 its nasal cavity, there, by renewed ecdysis, 

 throwing off its spiny covering, and finally 

 attaining the complete organisation of the 

 sexually-mature Pentastomum. But if no such 

 favourable opportunity is afforded the larva of 

 reaching its final host, it becomes re-encysted 

 within the body of the intermediate host. 

 Encysted larvae which are swallowed by a beast 

 of prey with the flesh of the host, and thus 

 reach the intestine of the former, if sufficiently 

 mature, break through the intestinal wall, and 

 by active locomotion reach the respiratory 

 tissues and the nasal cavity (Gerlach, Stiles). 



a- 



Fig. 63.— Free larva of 

 Pentastomum taenioides 

 (the so-called P. denti- 

 cvlatum), from the liver 

 of the rabbit or the 

 nasal cavity of the dog 

 (after Leuckart). a, 

 anus ; d, intestine ; ft, 

 hooks ; m, mouth ; st, 

 circles of spines ; tp, 

 tactile, papillae. 



3. General Considerations. 



The most important point in the ontogeny of Pentastomum is the 

 appearance of a larva furnished with two pairs of limbs. This 

 larval form distinctly indicates that in Pentastomum we have an 

 Arthropod, a fact which is not so evident from the organisation 

 of the adult. It was this larval form above all that led to the 

 classing of the Pentastomidae with the Acarina. The similarity 



