540 PENTASTOMIDA. 



In the sexual state it inhabits the nasal cavities of the dog. The early 

 embryonic development takes place as the ovum gradually passes down the 

 uterus. The segmentation appears to be complete ; and gives rise to an 

 oval mass in which the separate cells can hardly be distinguished. This 

 gradually differentiates itself into a characteristic embryo, divided into a tail 

 and trunk. The tail is applied to the ventral surface of the trunk, and on 

 the latter two pairs of stump-like unsegmented appendages arise, each 

 provided with a pair of claws. At the anterior extremity of the body is 

 formed the mouth, with a ventral spine and lateral hook, which are perhaps 

 degenerated jaws. The spine functions as a boring apparatus, and an 

 apparatus with a similar function is formed at the end of the tail. A larval 

 cuticle now appears, which soon becomes detached from the embryo, except 

 on the dorsal surface, where it remains firmly united to a peculiar papilla. 

 This papilla becomes eventually divided into two parts, one of which remains 

 attached to the cuticle, while the part connected with the embryo forms a 

 raised cross placed in a cup-shaped groove. The whole structure has been 

 compared, on insufficient grounds, to the dorsal organ of the Crustacea. 



The eggs, containing the embryo in the condition above described, are 

 eventually carried out with the nasal slime, and, if transported thence into 

 the alimentary cavity of a rabbit or hare, the embryos become hatched by 

 the action of the gastric juice. From the alimentary tract of their new host 

 they make their way into the lungs or liver. They here become enveloped 

 in a cyst, in the interior of which they undergo a very remarkable metamor- 

 phosis. They are, however, so minute and delicate that Leuckart was 

 unable to elucidate their structure till eight weeks after they had been 

 swallowed. At this period they are irregularly-shaped organisms, with a 

 most distant resemblance to the earlier embryos. They are without their 

 previous appendages, but the alimentary tract is now distinctly differentiated. 

 The remains of two cuticles in the cyst seem to shew that the above changes 

 are effected in two ecdyses. 



In the course of a series of ecdyses the various organs of the larval form 

 known as Pentastomum denticulatum continue to become differentiated. 

 After the first ( = third) ecdysis the cesophageal nerve-ring and sexually 

 undifferentiated generative organs are developed. At the fourth (=sixth) 

 ecdysis the two pairs of hooks of the adult are formed in pockets which 

 appeared at a somewhat earlier stage ; and the body acquires an annulated 

 character. At a somewhat earlier period rudiments of the external genera- 

 tive organs indicate the sex of the larva. 



After a number of further ecdyses, which are completed in about six 

 months after the introduction of the embryos into the intermediate host, the 

 larva attains its full development, and acquires a form in which it has long 

 been known as Pentastomum denticulatum. It now leaves its cyst and 

 begins to move about. It is in a state fit to be introduced into its final host ; 

 but if it be not so introduced it may become encysted afresh. 



If the part of a rabbit or hare infected by a Pentastomum denticulatum 

 be eaten by a dog or wolf, the parasite passes into the nasal cavity of the 



