382 E. RAY L/VNKESTER. 



phase with creeping foot, with mantle-flap and eye-tentacles, 

 the cilia no longer predominate on the velum, but it remains 

 as a well-marked ridge swelling out into a pair of lobes, one 

 on each side of the mouth, and terminating bluntly on each 

 side at the back of the head (PI. XVll, figs. 7 and 10). 



At the same time during the Veliger-period in which the 

 foot commences to assume a bilobed form, a conical eminence 

 appears on each side within the heart-shaped velar area. 

 These two eminences are the eye-tentacles, and rapidly grow 

 so as to overshadow the margins of the velum. In this phase 

 of the development, as the embryo rotates, it often presents 

 itself in the position seen in PI. XVII, fig. 6, in which the 

 foot is stretched in front, and the velar area with the growing 

 eye-tentacles, and the mouth placed centrally, complete the 

 rest of the visible part of the Veliger. The dark coloured 

 margin of the velum itself is seen forming a curious saddle- 

 shaped cincture placed transversely. It is easy enough to 

 demonstrate that the velum actually persists in adult life by 

 comparing such embryos as figs. 7 and 10 with full-grown 

 Lymnsei. The fact that some of the Pulmonata thus retain 

 this larval organ in the adult condition is important, be- 

 cause, so far as I know, no other mollusc has been shown 

 to do so ; and, in fact, no other organism which possesses a 

 velum in its younger phases of development, such as the 

 Echinodermata, Nemerteans, Gephyreans, and Chsetopodous 

 Annelids, with the exception of the Rotifers. Parts of the 

 prostomial region in some of the Chsetopodous Annelida may 

 perhaps be traceable to the larval velum, as are the sub- 

 tentacular lobes of Lymnceus. 



The retention of the velum and the strongly bilobed cha- 

 racter of the young foot mark the Pulmonata as an archaic 

 group of odontophorous Mollusca. The presence of these 

 archaic features is in accordance with the generalisation that 

 such features may be looked for in the fresh-water repre- 

 sentatives of large sea-dwelling groups, other examples being 

 found in the fresh-water Radiolaria, in Hydra and Cordy- 

 lophora, and in the living Ganoid fishes. 



Nerve-ganglion. — Within the velar area coincidently with 

 the commencing development of the eye-tentacles a bilobed 

 mass of cells commences to develop, apparently from a local 

 multiplication of cells belonging to the epidermic layer. 

 They form a conspicuous mass, and enter into ponnection 

 with the pharyngeal mass (figs. 8, 17, 23 ng). This is the 

 supra-oesophageal nerve-mass, and it is to be noted that its 

 mode of development is identical with that which I have 

 elsewhere described in Aplysia, 



