862 CLASS V. ARACHNIDA. 



vesiculae pass into a .chitin-lined vas deferens which receives 

 the contents of large backwardly directed ejaculatory glands, 

 one on each side. The vasa deferentia then traverse a cirrus 

 bulb which receives on one side a cirrus-sac and on the other 

 a dilator-rod-sac, both of which assist in copulation. The two 

 ducts then unite and open by a median ventral pore a little 

 posterior to the mouth. 



The ovary is also a median dorsal sac, which splits anteriorly 

 into a right and left oviduct, which pierce the hook-gland and 

 encircle the stomach and the ninth pair of nerves. The oviducts 

 unite ventrally and at their point of union receive the ducts of 

 two receptacula seminis or spermathecae which as a rule con- 

 tain numerous spermatozoa. The uterus formed by the united 

 oviducts is continued along the ventral surface of the body as 

 a coiled tube often crowded with fertilized eggs ; as many as 

 half a million may be found. The uterus opens by a narrow 

 vagina in the median ventral line a little in front of the anus. 



The ova undergo a considerable part of their development 

 within the body of the mother. In those forms which live in 

 the nasal passages, the eggs when laid leave the host with its 

 nasal excretions ; but in those which inhabit the lungs, the eggs 

 pass down the alimentary canal and leave the host's body with 

 the faeces. For their further development it is necessary that 

 they be eaten by some other animal, and this second or larval 

 host is as a rule herbivorous. The larva, which emerges when 

 the egg-shell is dissolved in the stomach of the second host, is 

 globular with a tail and two pairs of appendages ending in hooks. 

 Anteriorly is a boring apparatus of chitinous rods. By means 

 of this armature the larva bores through the stomach walls 

 and comes to rest in some organ, often the liver. Here a cyst 

 is formed in which it lies. It moults a few times and loses its 

 limbs, but its internal organs are maturing and the annulations 

 appear, arising first about the middle of the body. After a 

 quiescent period of some months the larva again becomes active 

 and wanders through the tissues of its second host, a proceeding 

 which is sometimes fatal to the latter. If the second host should 

 now be eaten by a member of the first, i.e. some carnivorous 

 creature, the larva makes its way into the nasal passages or 

 lungs as the case may be, and after a final moult becomes adult. 



The larval form with its two pairs of clawed limbs recalls the 



