Dr. \V. Saleusky on lliickeVs Gastraea Theoi-i/. 11 



elucidated by the three figures already given (PI. V. figs. 1-3), 

 which represent three characteristic developmental phases of 

 the oyster. 



But as regards the Cephalo])hora, the greater part of the ob- 

 servations on this class of the Mollusca agree in showing that, 

 after the segmentation, the i^^^^ of these animals becomes con- 

 verted into a body which consists of two different elements — 

 namely, coarsely granular, which lie in the interior of the em- 

 bryo, and paler, which surround the preceding. Such deve- 

 lopmental stages have been demonstrated in the Pteropoda 

 {Tiedanannia and Cavolinia) and Heteropoda [Pterotrachea 

 coronata) by the very complete and remarkable observations 

 of Gegenbaur* ; the same conditions are presented, according 

 to J. Miiller +, by Eiitoconcha mirabilis ; and Dentalium has a 

 similar development, according to Lacaze-Duthiers \. I have 

 myself described the same Planula-stnge occurring first after 

 segmentation in the Prosobranchiata {Cali/ptrcea, JVassa, and 

 Trochim^). In all the animals mentioned also the subsequent 

 phenomena occm* in a nearly concordant manner. First the 

 organs of locomotion are formed, then the foot ; the mouth and 

 oesophagus are invaginated, and finally the intestine is formed. 



AmjmUaria (according to Semper), Ancylus (according to 

 Stephanoft"), and Limnaus (according to Lereboullet) are deve- 

 loped somewhat differently from these Mollusca. If we com- 

 pare the statements of these last-mentioned naturalists, we 

 arrive at the conviction that the Gastrula-stage occurs only in 

 tlie ontogeny of Limnceus, according to the observations of 

 Lereboullet ij. But these observations are opposed by the 

 very recent beautiful observations of Ganin W (which unfor- 

 tunately are published without figm-es). From these last it 

 appears that the invagination of Limnceus does not correspond 

 to those oiAmphioxus^ the Ascidia, &c., but is rather homolo- 

 gous with the invagination of embryos of Calyptrcea^ which 

 in CaJyptnea separates from each other the rudiments of the 

 vela, foot, and cephalic vesicle. At the bottom of this invagi- 

 nation, in Limmvus as in Calyptrcea^ the oesophageal invagi- 

 nation is formed. 



In the Cephalopoda there can certainly be no question of a 

 Gos^rw/a-stage. 



* Untersuchungen uber die Pteropoden und Heteropoden. 



t Ueber Synaptn digitata, und liber die Erzeugung der Schnecken in 

 Ilolotliiirien. 



X Memoirs in Ann. des Sci. Xat. 1854-57. 



§ Zeitschr. fiir ^nss. Zool. Bd. xxii. 



II Recherches sur le d^veloppement de la truite, du lezard et de la 

 lininee. 



il Waischauer Universitats-Nachrichten ; also Nitsclie's Reports, 1872. 



