Dr. W. Salensky on Hackel’s Gastrea Theory. 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 Cephalophora, the greater part of the ob- 
servations on this class of the Mollusca agree in showing that, 
after the segmentation, the egg 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 
(Tiedemannia and Cavolinia) and Heteropoda (Pterotrachea 
coronata) by the very complete and remarkable observations 
ot Gegenbaur* ; the same conditions are presented, according 
to J. Miiller +, by Kntoconcha mirabilis ; and Dentalium has a 
similar development, according to Lacaze-Duthiers f. I have 
myself described the same Planula-stage occurring first after 
segmentation in the Prosobranchiata (Calyptrea, Nassa, and 
Trochus§).  Inall the animals mentioned also the subsequent 
phenomena occur in a nearly concordant manner. First the 
organs of locomotion are formed, then the foot ; the mouth and 
cesophagus are invaginated, and finally the intestine is formed. 
Ampullaria (according to Semper), Ancylus (according to 
Stephanoff), and Limneus (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 
the ontogeny of Limneus, according to the observations of 
Lereboullet ||. But these observations are opposed by the 
very recent beautiful observations of Ganin {| (which unfor- 
tunately are published without figures). From these last it 
appears that the invagination of Limneus does not correspond 
to those of Amphioxus, the Ascidia, &c., but is rather homolo- 
gous with the invagination of embryos of Calyptrewa, which 
in Calyptrea separates from each other the rudiments of the 
vela, foot, and cephalic vesicle. At the bottom of this invagi- 
nation, in Limneus as in Calyptrea, the cesophageal invagi- 
nation is formed. 
In the Cephalopoda there can certainly be no question of a 
Gastrula-stage. 
* Untersuchungen uber die Pteropoden und Heteropoden. 
+ Ueber Synapta digitata, und iiber die Erzeugung der Schnecken in 
Holothurien. 
¢{ Memoirs in Ann. des Sci. Nat. 1854-57. 
§ Zeitschr. fiir wiss. Zool. Bd. xxii. 
|| Recherches sur le développement de la truite, du lézard et de la 
limnée. 
{| Warschauer Universitats-Nachrichten ; also Nitsche’s Reports, 1872. 
