422 M. Leuckart on the genus Sacculina. 



in cither direction. We also still want many important data, 

 necessary, and perhaps decisive, in the estimation of the con- 

 ditions now under consideration. Under such circumstances we 

 cannot emphatically enough recommend the continued careful 

 study of the Aphides. We must still admit the truth of the 

 concluding sentence appended by the acute De Geer to his 

 memoirs upon the Aphides : — " The Aphides are insects which 

 arc in a position to overthrow the entire supposed system of gene- 

 ration, and to confuse those who atteu)pt to investigate this 

 mystery of nature/^ 



Giessen, Sept. 1858. 



Postscript. 



In the course of the present summer I have been no more 

 successful than last year in discovering male Bark-lice, or even 

 detecting any trace of their existence. The only thing that I 

 can add to the preceding statements, is that the comparative 

 time of the evolution varies considerably according to external 

 circumstances. Whilst, last year, the second winged generation 

 made its appearance only in August, I observed it this year as 

 early as the end of June. 



Giessen, July 12, 1859. 



EXPLANATION OF PLATE VIL 



Fig. 1. Sexual apparatus of the winged female of Chermes Laricis. 



Figs. 2 & 3. Lower part of the single seyual passage of Chermes Ahietis, 



with the two lubricating glands. 

 Fig. 4. The same part in Phylloxera Quercus. 



XLII. — Observations on the genus Sacculina, Thompson (Pachy- 

 bdella, Diesing -, Peltogaster, Rathke). By R. Leuckart*. 



[With a Plate.] 



In his ^' Travelling Observations in Scandinavia f,^' and " Contri- 

 butions to the Fauna of Norway J,^' Rathke has described, under 

 the new generic name of Peltogaster, two species of a very pe- 

 culiar flat and sac-like parasite, which is attached, by means of 

 a " sucker-like structure," to the abdomen of the Paguri and of 

 certain Brachyurous Crustacea. Rathke regarded this remark- 

 able animal as a worm, but at the same time indicated (at least 

 in the anatomical description contained in the memoir first 

 quoted above) certain analogies with Cyclops and the Lernmce, 



* Translated from Wiegmann's 'Archiv,' 1859, p. 232, by W. S. Dallas, 

 F.L.S., Keeper of the York Museum. 



t Neueste Schriften der naturf. Gesellsch. in Danzig, 1842, Bd. iii. 

 Ueft 4, p. 105. 



X Verh. der K. L. C. Akad. Bd. xx. Abth. i. p. 245. 



