86 Mr. E. Ray Lankester's Zoological Observations 



development. It is in this way also that the "cuttle-bone" of 

 Loligo takes its origin ; and from the observation of this common 

 mode of origin of the shells of Lamellibranchs, Gasteropoda, 

 and cuttlefish, I do not doubt that they are fundamentally 

 identical or homogenous — that is to say, have a common an- 

 cestral representative. The pharynx and oesophagus early 

 develop in Aplysia as in-pushings at the opposite pole to that 

 at which the shell-gland appears, which latter is the pole of 

 active segmentation in the first embryonal changes. 



The supraoesophageal ganglion is clearly seen to develop as 

 a thickening of the outer layer of cells in the prostomial region. 

 It sends branches downwards and forwards, and gave rise to 

 the suspicion that the suboesophageal nervous mass was but a 

 lobe of it. 



Below the mouth, in a blunt process (which is the foot) the 

 pair of otolithic sacs (or otocysts, as M. de Lacaze-Duthiers 

 terms them) appear ; I took great pains to ascertain their 

 earliest beginning. They certainly never communicate with the 

 exterior ; they have been erroneously supposed to do so in 

 Gasteropoda ; and I have established the fact that they really do 

 so in Cephalopoda. The first appearance of each otocyst is, 

 before any organs except the shell-gland are indicated, as a 

 faint vesicle, with no proper walls of its own, just below the 

 most superficial layer of cells ; and I believe that it really be- 

 longs to that layer. As the foot develops, the otocyst shifts 

 greatly its position, and acquires thicker walls and larger size. 

 Tlie otolith develops within the cyst at a late period ; often it 

 may be seen in one cyst and not in the other. 



Development of Nudibranchs. 



The eggs of species of Doi-is, of Tethys, Pleurobranchus, and 

 others were frequently studied. I found those of Polycera 

 quadrilineata and oiEolis exigiia the most favourable for study. 

 I was able to determine in these that the first step in develop- 

 ment, after the formation by cleavage of the mass of embryo- 

 cells or "polyblast," is the invagination or in-pushing of these 

 cells at one pole, just as Kowalewsky has drawn it in Amphi- 

 oxus and Phallusia, and as seen also in the Heteropod mollusk 

 Atalanta. The orifice of invagination is at one time large and 

 obvious enough, but closes entirely at a very early period. 

 The same invagination and orifice I have made out in the 

 Lamellibrancli Pisidium, the development of which I studied 

 in the spring of 1871 at Jena. I also observed it in Limax'j 

 and its occurrence in a similar stage in certain marine Lamel- 

 libranchs is clear from Lov^n's admirable figures, though he 

 has nristaken its significance. 



