468 INVERTEBRATE MORPHOLOGY. 



Affinities of the Tardigrada. The embryological history of these 

 forms has not yet been sufficiently studied to allow of any definite conclu- 

 sions as to their affinities. The presence of four pairs of limbs has usually 

 been regarded as pointing to a relationship with the Acarina, but the ab- 

 sence of all mouth-appendages, the structure of the legs, and the position 

 of the last pair with regard to the anal opening, not to mention the peculi- 

 arities of the internal organization, are opposed to any close relationship 

 with the Arachnida. The Tardigrada must be considered as holding an 

 independent position, without distinct indications of relationship with any 

 of the types, until further information as to their developmental phenomena 

 has been secured. 



LITERATURE. 



PENTASTOMIDA. 



R. Leuckart. Ban und EntwicklungsgescJiichte der Pentastomen. Leipzig uud 

 Heidelberg, 1860. 



C. W. Stiles Ban und Entwicklungsgeschichte wn Pentastomum proboscideum 

 und P. subcylindricum. Zeitsckr. filr wissensch. Zoologie, LII, 1891. 



A. B. Spencer. T7ie Anatomy of Pentastomum teretiusculum (Baird). Quar- 

 terly Journal of Microscop. Science, xxxiv, 1892. 



PYCNOGONIDA. 



E. B. Wilson. TJie Pycnogonida of New England and Adjacent Waters. Re- 

 port of the U. S. Commissioner of Fish aud Fisheries for 1878. Wash- 

 ington, 1880. 



A. Dohrn. Die Panlopoden des Golfes von Neapel. Fauua und Flora des 

 Golfes von Neapel. Monographic, in, 1881. 



P. P. C. Hoek. Report on the Pycnogonida. Scientific Results of the Voyage 

 of H.M.S. Challenger. Zoology, in, 1881. 



T. H. Morgan. A Contribution to t7te Embryology and Phytogeny of the Pycno 

 gonids. Studies for the Biol. Laboratory, Johns Hopkius University, v, 

 1891. 



TARDIGRADA. 



L. Plate. Beitrdge zur Naturgeschichte der Tardigraden. Zoolog. Jahrbiicher, 

 Anatom. Abtheilung, in, 1888. 



