PENTASTOMIDA. 539 



second pair of appendages becomes enlarged again and forms the so-called 

 palpi, while the third pair develops in the male into the egg-carrying append- 

 ages, but is aborted in the female. The first pair form appendages lying 

 parallel to the rostrum, which are sometimes called pedipalpi and some- 

 times antennas. 



The anal papilla is a rudimentary abdomen, and, as Dohrn has shewn, 

 contains rudiments of two pairs of ganglia. 



The larva? of Phoxichilidium are parasitic in various Hydrozoa (Hydrac- 

 tinia, etc.). After hatching they crawl into the Hydractinia stock. They 

 are at first provided with the three normal pairs of larval appendages. The 

 two hinder of these are soon thrown off, and the posterior part of the trunk, 

 with the four ambulatory appendages belonging to it, becomes gradually 

 developed in a series of moults. The legs, with the exception of the hinder- 

 most pair, are fully formed at the first ecdysis after the larva has become 

 free. In the genus Pallene the metamorphosis is abbreviated, and the 

 young are hatched with the full complement of appendages. 



The position of the Pycnogonida is not as yet satisfactorily settled. 

 The six-legged larva has none of the characteristic features of the Nauplius, 

 except the possession of the same number of appendages. 



The number of appendages (7) of the Pycnogonida does not coincide 

 with that of the Arachnida. On the other hand, the presence of chelate 

 appendages innervated in the adult by the supra-cesophageal ganglia rather 

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

 shewn above (p. 455) all the appendages in the embryo of true Arachnida 

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

 in the larvae 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 siiperiori in Firenze, 1877. 



(536) An. Dohrn. " Ueber Entwicklung u. Baud. Pycnogoniden." Jenaische 

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

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



(537) G. Hodge. " Observations on a species of Pycnogon, etc." Annul, and 

 Mag. of Nat. Hist. Vol. IX. 1862. 



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

 Larvenformen." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. I. 1874. 



PENTASTOMIDA. 



The development and metamorphosis of Pentastomum tsenoides have 

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

 for the group. 



