PHYSIOLOGY OF THE INVERTEBRATA. 439 



The Aeachnida. 



In the Pentastomida, the ovary of the female is a large 

 sac-like organ, with oviducts which pass off from its anterior 

 end. The oviducts terminate in an aperture situated near 

 the anus. The ova are developed in the ovary. The testis of 

 the male is situated on the ventral side of the intestine. It 

 is provided with two vasa deferentia, which pass in an 

 anterior direction, and terminate in two dilatations, which 

 contain long, chitinous penes {i.e., a penis in each dilatation). 

 These animals are parasitic, and the parasitism is almost 

 similar to that of the Cestoidea — e.g., Pentastovium denticula- 

 tum, which is sexless, inhabits the liver of rabbits and hares ; 

 but in the sexual state this parasite, known as Pentastomwm 

 tcenioides, infests the nasal cavities of wolves and dogs. 



In the Arctisca or Tardigrada, the sexes are not distinct, 

 for these microscopic animals are hermaphrodites. The 

 ovary is a sac-like organ, situated on the posterior half of 

 the digestive canal, and opens into the cloaca, which is a 

 dilatation of the rectum. The ova of JSmydium, Milnesium, 

 and Macrdbiotus ursellus are invested by a chorion, and they 

 are deposited in an ephippium, which is in reality the cuticle 

 of the parent. The vitellus undergoes complete segmentation, 

 but there is no metamorphosis.* 



The Pycnogonida are dioecious, and "the testes and ovaria 

 are lodged in the legs and open upon their basal joints. The 

 embryo emerges from the egg as a larva provided with a 

 rostrum, and with three pairs of appendages, which represent 

 the short anterior three pairs in the adult. (A. Dohrn.) The 

 four pairs of great limbs of the adult are produced by 

 outgrowths from a subsequent posterior elongation of the 

 body." 



In the Acarina, the sexes are distinct. The male organs 

 are formed on distinct types. The testes of Ixodes, for 



* See Kaufmann's paper in Zeitschrift fiir I17s«. Zoologie, 1851. 



