86 



SECTIONAL ADDRESSES. 



reveal the original larval type of a less specialised Zygobranchiate 

 Gastropod. 



But the fully developed post-torsional Veliger of au ordinary Azygo- 

 branch is thoroughly adapted to an active pelagic career. Its prototroch 

 having now grown out into a pair of velar lobes, the larva no longer rotates 

 like a trochosphere, but directs its movements up, down, or straight ahead 

 on a perfectly even keel. Its velum is so powerful that it can easily 

 sustain the added weight of its partly calcified shell. When suddenly 

 disturbed it reacts in characteristic fashion : its head and velar lobes are 

 immediately withdrawn into the now adjacent gill-cavity, the foot smartly 

 follows suit, and the door is automatically closed by its horny operculum. 

 Owing to its weight the larva falls vertically downwards in the water the 

 moment it stops swimming. As an obscure prae-Georgian poet has some- 

 where described it : — 



' The Veliger's a lively tar, the liveliest afloat ; 

 A whirling wheel on either side propels his little boat ; 

 But when the danger signal warns his bustling submarine, 

 He stops the engine, shuts the port, and drops below unseen.' 



A 



B 



Fig. 5.— Veligers of Azygobranchs. 

 A, Nassa ; B, Dolium ; C, Opisthobranch. 



Now il 1 have succeeded in my description of the principal points, I 

 think you will agree that whatever significance may be attached to the 

 twist of its body in an adult Gastropod, there is no doubt about its value 

 to the larva. As to the origin of this torsion, all previous attempts to 

 explain it have been based on the assumption that it arose during the 

 adult life of some early type of Mollusk. I do not propose to go into all 

 these theories, instructive as they are, since their divergences merely 

 illuj^trate the difficulty of any solution on those lines. Like the asymmetry 

 of Amphioxiis and the one-sided preponderance of Echinoderms, the 

 torsion has remained a standing puzzle. It has been hardly attempted 

 to assign a utilitarian value to the initial and intermediate stages which 

 must have been required to effect the change in a series of adult ancestors. 

 I will deal later with the further point as to the failure of any of these 

 intermediate links to persist. 



