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
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