D.— ZOOLOGY. 81 



attains its full size and is often arrested in a very vestigial condition. 

 Finally, the pelagic stage may be suppressed altogether, and the Whelk 

 emerges from the confinement of its brood-chamber as a diminutive 

 adult, ready at once to pursue its definitive career. 



The absence of any larval stage throughout the whole class of 

 Cephalopoda is doubtless due to the locomotive agility of the adult which 

 renders a distributive larval phase unnecessary. Although this explana- 

 tion applies to few other cases of suppression, the fact seems from one 

 standpoint to furnish the climax of the evolutional sequence we have 

 been considering. For the larval phase, like the seed of a plant, is 

 essentially distributive, and in the evolution of Mollusca we have to some 

 extent seen it shift along the steps of the life-history from a very early, 

 simply organised, shell-less stage, the Trochosphere, to an intermediate 

 shell-bearing stage, from this to the highly adapted Veliger or Rotiger, 

 and finally (if we may here include the Cephalopod and the Whelk) to the 

 adult stage itself, the lower stages of development having been successively 

 relegated to the embryonic period. Broadly speaking, this sequence 

 corresponds with an increase in the yolkiness of the eggs, a very simple 

 and widely distributed means of postponing the hatching period to a more 

 advanced stage of development. 



It is probably not without significance that this progressive shift 

 corresponds with a time-sequence observable in the order of appearance 

 of the groups concerned, the groups with free Trochospheres, viz. 

 Zygobranch Gastropods and Protobranch Bivalves dating from the 

 Lower or Middle Cambrian, while the groups with Veliger and Rotiger 

 larvae, the Pectinibranchs, Opisthobranchs, and Eulamellibranchs, appear 

 to be unknown before the late Silurian. A curious exception, urgently 

 calling for further investigation, is the alleged occurrence of Capulids in 

 the Lower Cambrian. 



Although, with fuller knowledge of the facts and of the bionomical 

 conditions, it may be possible to explain the cases of reduction or oblitera- 

 tion of the larval stage in terms of adaptation, it seems more probable 

 that there has been a secular change tending to depreciate the value of 

 dispersal as the seas became stocked with an increasing number and 

 variety of specialised inhabitants. When the adults have become highly 

 adapted to the conditions of a particular kind of terrain {e.g. rock-life) a 

 prolonged larval life would be of doubtful advantage which regularly 

 carried a large percentage of the larvae away from the rock zone altogether 

 and landed them in an area of sand and mud. 



On the other hand we cannot overlook Prof. Tattersalls Littorina,' 

 which, not content with all the conventional larval stages, has started a 

 new distributional device of its own by setting adrift the egg-case as well, 

 remarkably adapted to that end. 



It is with larval origins, however, not suppressions, that I am now 

 concerned. To some zoologists this question does not arise, or at least 

 presents no serious difficulties. With them larval stages represent fore- 

 gone ancestors, and all they have to do is to account for discrepancies. 

 As the chain of adult ancestors is drawn out, at each new evolutional 



■'■ Littorina, Tattersall, Fisheries, Ireland, Sci. Invest., 1920, 1- 

 1928 G 



