NOTES ON THE DEVELOI'M KN T OP MOLLUSCA. 367 



Notes on the J)evelopment of Mollusca. By Alfred C. 

 Haddox, M.A.^ Professor of Zoology, Royal College of 

 Science, Dublin. (With Plate XXXI.) 



With the exception of the investigation on Purpura, all the 

 following notes were made in the spring of 1879, while I was 

 occupying the table belonging to the University of Cambridge in 

 Dr. Dohrn's Zoological Station at Naples. I would here express 

 my thanks to the University, and, at the same time, to the 

 officials of the Zoological Station for their uniform kindness to 

 me. 



The following notes are purposely fragmentary, as I do not 

 wish to lengthen my paper by needless repetitions of other 

 people's work. It is, however, but fair to myself to state that I 

 have made an immense number of observations, preparations,- 

 and drawings, more or less of which corroborate the researches 

 of the numerous previous investigators of MoUuscan Embryo- 

 logy. 



Nudibranchiata. — The eggs of Elysia viridis are extremely 

 favorable for showing the earliest developmental phases, such 

 as the male and female pronuclei (impregnation), constriction 

 of the polar cells and the concurrent well-marked amoeboid 

 movements of the egg, radial striae and other phenomena of seg- 

 mentation, all of which can be readily studied in the living 

 ovum. 



In the majority of the Nudibranchs the segmentation is very 

 regular, resulting in the formation of a well-marked gastrula, 

 which is usually formed by a kind of rolling over combined with 

 invagination. The extreme of the former process of gastrula: 

 formation is apparently reached in the Nematoid Cucullanus} 

 The slit-like blastopore closes over from behind forwards, and 

 I have reason to beheve that in Fiona (sp. ?) at least the blas- 

 topore either becomes the permanent mouth, or the latter is in- 

 vaginated at the spot where the former finally closes up. 



At the commencement of the Veliger-stage two large cilia 

 make their appearance in the centre of the velar area.^ There 



1 Butschli, 'Zeit. f. Wiss. Zool.,' xxvi, 1876. 



^ An anticipatory notice of this and of the succeeding observation will 



