of Lamellaria perspicua. 121 



united by a commissure which becomes shorter and shorter. 

 The eyes are formed at the inferior angle of these inflations, 

 at the expense of the ectoderm ; their development progresses 

 pari passu with that of the subcesophageal nervous centres j 

 at the time of hatching they enclose two refractive vesicles. The 

 otocysts appear at the base of the foot at the time of the 

 formation of the latter, and before the existence of any 

 nervous organ ; their wall is composed of very small cells 

 belonging to the ectoderm. 



As soon as the stomach is differentiated at the expense of 

 the endoderm, its cavity and the lumen of the oesophagus are 

 lined with very delicate vibratile cilia. At the same stage 

 we see, on the right side of the embryo, a rounded mass of 

 large cells, which will form the kidney. The residue of the 

 endodermic spheres not differentiated is pressed back to the 

 lower extremity of the embryo, and gives origin, not to the 

 liver, which originates from the stomach, but probably to the 

 genital organs. I have not been able to follow the formation 

 of these last organs or of the anal gland, which is much deve- 

 loped in the adult Lamellaria. 



The cavity of the mantle is formed by a very rapid deve- 

 lopment of the secretory pad of the shell. The pallial con- 

 tour is pigmented with brown and yellow. The dorsal part 

 of the mantle is finely ciliated. Above the digestive tube and 

 along the lower part of the foot we find some contractile sinuses, 

 the first indications of a circulatory system. 



The preconchylian invagination, the general importance of 

 which in the Mollusca was first pointed out by Ray Lankester, 

 is not so strongly marked in Lamellaria as in certain nudi- 

 branchs (Dendronotus arborescens 7 Ooniodoris nodosa) in 

 which I have had the opportunity of observing it. We see in 

 the lower part of the embryo, at that stage when the cephalic 

 vesicle begins to be differentiated, the ectoderm become hollow 

 very slowly and leave a thin cuticle free, which is the rudiment 

 of the first shell. The cushion which borders this invagina- 

 tion ascends by degrees along the embryo, in the same way as 

 a wave of liquid propagates itself, at the same time that the 

 bottom of the invagination resumes its original form and 

 position. The thickness of the cushion keeps the embryo 

 aAvay from the shell ; and the ectodermic cells continuing their 

 secretion, a second shell is formed inside the first, but closely 

 applied to the body of the embryo. The first shell is of a 

 nautiloid form, and presents two dorsal and two lateral keels; 

 it resembles the shell of Atlanta. The second shell is more 

 simple, and resembles in its appearance that of Carinaria 1 



