PENTASTOMIDA. 449 



points to a common phylum for the Pycnogonida and Arachnida ; though as 

 shewn above (p. 376) all the appendages in the embryo of true Arachnida 

 are innervated by post-oral ganglia. The innervation of these appendages 

 in the larva? of Pycnogonida requires further investigation. Against 

 such a relationship the extra pair of appendages in the Pycnogonida is 

 no argument, since the embryos of most Arachnida are provided with four 

 such extra pairs. The two groups must no doubt have diverged very 

 early. 



BIBLIOGRAPHY. 



(535) G. Cavanna. " Studie e ricerche sui Picnogonidi." Pubblicazioni del R. 

 Institute di Studi superior! in Firenze, 1877. 



(536) An. Dohrn. "Ueber Entwicklung u. Bau d. Pycnogoniden." Jenaisclie 

 Zeitschrift, Vol. v. 1870, and "Neue Untersuchungen lib. Pycnogoniden." Mittheil. 

 a. d. zoologischen Station zu Neapel, Bd. i. 1878. 



(5^7) & Hodge. "Observations on a species of Pycnogon, etc." Annal. and 

 Mag.' of Nat. Hist. Vol. ix. 1862. 



(538) C. Semper. "Ueber Pycnogoniden u. ihre in Hydroiden schmarotzendeu 

 Larvenformen." Arbeiten a. d. zool.-zoot. Jnstit. Wiirzburg, Vol. i. 1874. 



PENTASTOMIDA. 



The development and metamorphosis of Pentastomum tsenioides have 

 been thoroughly worked out by Leuckart (No. 540) and will serve as 

 type for the group. 



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 appa- 

 ratus, 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 metamorphosis. 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 



B. E. 29 



