MOLLUSCOUS ANIMALS GENERALLY. 



185 



Nudilranchiate order generally (consisting of the Doris, Eolis, and other 

 'sea-slugs') it forms a long tube with a membranous wall, in which im- 

 mense numbers of eggs (even half a million or more) are packed closely 

 together in the midst of a jelly-like substance, this tube being disposed 

 in coils of various forms, which are usually attached to the Sea- weeds or 

 Zoophytes. The course of development, in the first and last of these 

 instances, may be readily observed from the very earliest period down to 

 that of the emersion of the embryo; owing to the extreme transparence 



FIG. 403. 



Embryonic development of Doris bilamellata:A, Ovum, consisting of enveloping membrane a 

 and yolk 6 ; B, c. D, E, F, successive stages of segmentation of yolk; o, first marking-out of the 

 shape of the embryo ; H, embryo on the 8th day; i, the same on the 9th day; K, the same on the 

 12th day, seen on the leftside at L; M, still more advanced embryo, seen at N as retracted within 

 its shell: a, superficial layer of yolk-segments coalescing to give origin to the shell; c, c, ciliated 

 lobes; d, foot; gr, hard plate or operculum attached to it; h, stomach; i, intestine; m, n, masses 

 (glandular ?) at the sides of the oesophagus; o, heart (?); s, retractor muscle (?;; t, situation of 

 funnel; v, membrane enveloping the body; x, auditory vesicles; y, mouth. 



of the nidamentum and of the egg-membranes themselves. The first 

 change which will be noticed by the ordinary observer, is the ' segmenta- 

 tion ' of the yolk-mass, which divides itself (after the manner of a cell 

 undergoing binary subdivision) into two parts, each of these two into 



