380 E. RAY LANKESTER. 



the same position and having something of the same rela- 

 tive size as the wheel-apparatus of a Rotifer. Moreover, 

 in the Veliger the foot takes on a large relative growth, 

 so as to form a projecting lohe ; at first it is simple, but soon 

 becomes, what is exceedingly important, bilobed. This 

 bilobed condition of the foot need but be carried a very little 

 further than it is in the Lymnseus-veliger, and we should 

 have the Pteropod-veliger, with fully developed velum and 

 two epipodial " wings," such as I had the opportunity of 

 examining last spring at Messina through the kindness of 

 Dr. Herman Fol. 



In fig. 1 some interesting features are exhibited which 

 happened to be unusually well- presented by the particular 

 specimen from which the figure is drawn. The letter g 

 indicates the spot at which the gastrula's orifice of invagina- 

 tion has closed up, and the delicate pedicle of tissue {p i) ex- 

 tending from this to the enlarged gastrula-endoderm-cells is 

 the " pedicle of invagination," precisely similar to the pedicle 

 formed in the same way in the Lamellibranch Pisidium (for 

 an account of which I must refer to the forthcoming volume 

 of the ' Philosophical Transactions '). The thickened super- 

 ficial tissue to the left of the closed orifice of invagination is 

 the shell-disc — the earliest commencement of the mantle area.' 

 This again will be seen from the woodcut (fig. 2) to have 

 its equivalent in other molluscs, and has, indeed, been 

 especially described by Paul Stepanof in his account of the 

 development of the pulmonate Ancylus Jluviatilis. The 

 further development of this region has, however, escaped him 

 and all other previous observers. It is as a pushing in from 

 this shell-disc that the shell-gland to which I have referred 

 in the introduction, and which is seen in PL XVII, figs. 11, 

 12, 13, 14, and 17, is developed. 



The Shell-gland. — The shell-gland — a name which suggests 

 itself merely from the position of the gland, and not from any 

 necessary functional connection with the formation of the shell — 

 was seen figured and described by Lereboullet in his account of 

 the development oiLymneeus. Lereboullet accurately described 

 it at one period of its growth as a hollow cone, truncated and 

 closed at its deeper extremity. He regarded it as the anal 

 portion of the alimentary canal, and consequently termed it 

 the " anal cone." From this it follows that he had failed 

 to detect, as his figures also show to be the case, the 

 " pedicle of invagination" and the true commencement of 

 the terminal part of the alimentary tract. If we follow the 

 shell-gland through the various figures on PL XVII, in which, 

 it appears, we shall find that occupying at one time a very 



