THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY 



[SECOND SERIES.] 

 No. 93. SEPTEMBER 1855. 



XIII. — Observations on the Genera Pachybdella {Diesing) and 

 Peltogaster [Rathke), two animal forms parasitic upon the 

 abdomen of Crabs, By Professor Steenstrup*. 



In the most recent systematic work on the Worms, Intestinal 

 Worms and the lower division of the great Articulated series in 

 general, Diesing^s ^ Systema Helminthum^ (1850, vol. i. p. 434- 

 435), we find amongst the Bdellidea, in the suborder Monoco- 

 tylea, subtribe Cephalobdellida, a new genus Pachybdella, esta- 

 blished upon a parasitic animal discovered some years before 

 by Rathke under the abdomen of the common Crab, Carcinus 

 Manas, and described by him under the name of Peltogaster 

 Carcinif. Diesing himself appears to have had no specimens of 

 the animal before him, but to have merely drawn up his generic 

 and specific characters from Kathke's descriptions and figures, 

 which again are founded upon two specimens of this remarkable 

 parasite, one of which was obtained from the Norwegian coast, 

 the other from the Black Sea. Neither Rathke's description 

 nor his figures, however, furnish a sufficient notion of the 

 structure of the animal to enable us to arrive at any definite 

 conclusion as to its systematic position, or as to the group, 

 whose characteristic marks it might have lost in consequence of 

 its parasitic existence. It is only from the circumstance that 

 Rathke refers the animal to the genus Peltogaster, which had 

 been recently established by him, and of which he has more 

 fully described another species J {P.Paguri, Rathke), from the ab- 

 domen of the Hermit Crab, that we learn that this author would 



* From Wiegmann's Archiv. 1855, No. 1. p. 15. 



t Nova Acta Acad. Cses. Leop. Car. 1843, torn. xx. pt. 1. pp. 244-249. 

 X Partly in the same paper, p. 245-247, and partly in the Neuest. Schr. 

 der Nat. Ges. in Danzig, 1842, Bd. ii. p. 105-111. 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. xvi. II 



