INDUSTKIAL PROGRESS DURING THE YEAR 1875. clxxxi 



two or three weeks, the eggs being laid in February and 

 March. The embryo passes through an invaginate-gastrula 

 condition, as in certain nudibranch mollusks {Dendronotus 

 and Goniodoris). The first embryonic, deciduous shell is of 

 a nautiloid form, and presents two dorsal and two lateral 

 keels, like the shell of Atlanta. The second shell is more 

 simple, like that of Carinaria, or of the embryos of the Nu- 

 dibranches. Giard claims that they have the same relations 

 to each other, and the same relative signification as the nau- 

 plian cuticle of the embryos of the Cirripedes and the cara- 

 pace of the Archizcea inclosed under that cuticle. He does 

 not think that the second shell is the origin of the calcare- 

 ous shell of the adult Lamellaria. 



A beautiful memoir on the early stages of the Pteropods 

 and Heteropods, with admirable illustrations, has been pub- 

 lished by Fol in Lacaze-Duthiers's " Archives." 



The life-history of certain mollusks, abstracted from the 

 works mostly of European observers, has been given by Dr. 

 Packard in the American Naturalist. Of the mode of de- 

 velopment of the oyster we have much new information by 

 Salensky (1874) and Gerbe (1875). It appears that while 

 some lamellibranch mollusks, such as the Unio, are bisexual, 

 the oyster is hermaphroditic. The eggs, which are yellow, 

 after leaving the ovary are retained among the gills. A 

 single oyster may lay 2,000,000 eggs. The spawning time 

 of the oyster in Europe is from June to September. During 

 their development the eggs are inclosed in a creamy slime, 

 growing darker as the "spat" (the term applied to the 

 young oyster) develops. 



The course of development is this : After the segmen- 

 tation of the yolk, the germ divides into a clear peripheral 

 layer, and an opaque inner layer containing the yolk, and 

 representing the inner germinal layer. A few filaments or 

 large cilia arise on what is to form the " velum," or the future 

 head. The shell then begins to grow out at what is des- 

 tined to be the posterior end of the germ, and before the di- 

 gestive cavity arises. At this stage the two-layered germ 

 is said by Salensky to represent the "planula" of the 

 sponge. The digestive cavity is next formed (" gastrula " 

 stage), and the anus appears just behind the mouth, the ali- 

 mentary canal being bent at right angles. Meanwhile the 



