30 M. de Quatrefages on the Organization of 



digestive cavity in some Hirudines and Planarim. In the genera 

 Pelta and Chalidis no ramified appendages are found, but only 

 two large sacs, into which the alimentary substances enter and 

 remain for some time. 



The nervous system is also less perfect than in the ordinary 

 Gasteropoda, and approximates the Tunicata ; the postoesophageal 

 or ventral ganglia, and the transverse commissure uniting them 

 and completing the oesophageal ring posteriorly, being frequently 

 wanting, as are likewise the labial ganglia. 



For the reception of these peculiarly-organized Gasteropoda, 

 M. Quatrefages proposes the establishment of a new order in 

 that class, to be called Phlebentera, and which, with the genera 

 already mentioned, must include the genus Actteon, confounded 

 hitherto with the Aphysians, and, in all probability, Glaucus, the 

 Placobranchiata, and all other Gasteropods deprived of lungs and 

 vascular branchiae. Lastly, certain Planaria may perhaps be in- 

 serted under the same group. 



M. Quatrefages has also given to science a most instructive 

 memoir upon those polyps which, under the form apparently of 

 rugose amorphous crusts, are frequently found upon the whelk- 

 shells inhabited by the Paguri or hermit-crabs ; the species had 

 however always been confounded with the Hydra squamata of 

 Miiller, and neither its structure or mode of reproduction had 

 been studied. These polyps, designated by our author under 

 the name of Synhydra parasita, live attached by their base to a 

 common laminiform floor supported internally by a corneous net- 

 work and analogous to the polypary of the Gorgonia, but of a 

 more simple structure, resembling that of the skeleton of the 

 sponge. Thus associated simply in colonies by their bases, we 

 might suppose that each individual polyp exercised its functions 

 independently of another ; but they are in fact all united by a 

 system of capillary canals lodged deeply within the common ba- 

 silar tissue, and which establish ready communications between 

 their respective stomachs. 



The same arrangement for rendering the alimentary matters 

 digested by a single polyp available to the nutrition of the entire 

 colony may be observed also in the Alcyonia, the Corallinea, the 

 Gorgonia, Cornularia, &c, but previous to the discovery of M. 

 Quatrefages was unknown to occur in the Hydras. Another 

 particularly interesting fact is the singular structure of a certain 

 number of these polyps thus united in a kind of tuft. The one 

 kind present the usual form of Hydras, having a mouth sur- 

 rounded by filiform tentacles, so that they can directly obtain 

 food ; while the others are destitute both of oral orifice or appen- 

 dages, and depend solely for nutrition upon the products of di- 

 gestion in the former being conveyed to them by the system of 



