78 SECTIONAL ADDRESSES. 



gills !), Nucula and Yoldia to the lowest section of Bivalvia (Proto- 

 brancliia). But in the course of their career as free larvse, these 

 Molluscan trochospheres all acquire new and divergent features : the 

 trochosphere of Chiton lengthens out and develops a dorsal series of 

 cuticular, partly calcified, plates ; that of the Limpet acquires a shell 

 which is successively plate-like, cap-like, and nautiloid, its body under- 

 goes the Gastropod torsion, and it then develops an operculum ; the 

 Dentalium trochosphere develops a pair of mantle-folds and a saddle- 

 shaped shell, t which becomes tubular by ventral concrescence of its 

 edges ; the larval Yoldia acquires a hinged bivalved shell, and both it, 

 Dentalium, and Patella, but not Chiton, develop a foot. 



A B B 



Fig. 1. — Larvse of Chiton. 

 A, G. marginatus ; B, C. polii. 



It is readily Been that almost all "these characters which the trocho- 

 spheres acquire during their pelagic free-swimming career are in the 

 direct line towards their respective adult characters. As soon as the 

 rudiments of the shell have made their appearance, the larva of Nucula is 

 definitely a Bivalve, that of Dentalium a prse-Solenoconch, that of Patella 

 a Univalve, and that of Chiton Polyplacophorous. The secondary 

 characters which appear are essentially adult characters in the making. 

 They have mostly no relation to a pelagic career (e.g. the shell-plates of 

 a Chiton larva), and may even be an encumbrance — witness the useless 

 digging foot of the Dentalium larva — yet they appear. They can also be 

 no heirlooms from pelagic ancestors, since shell and foot speak unequivo- 

 cally of the ground— the archi-Mollusk was a benthic, not a pelagic 

 animal. These secondary larval characters then are mainly anticipations 

 of adult characters. 



But they are not entirely of this nature, for among the examples 

 mentioned the larva of the Limpet develops an operculum which is not 

 present in the adult stage. The early trochospheres of Dentalium and 

 Yoldia also show features which are both absent in the larva of Chiton and 

 have no direct relation to their adult characters. Let us examine these 

 cases a little more closely. 



The trochosphere of Chiton has a simple prototroch consisting of two 

 parallel rows of cells. As its body elongates the rudiments of six shell- 

 plates arise behind the prototroch, apparently in metameric order from 

 before backwards. During the pelagic career of the larva these plates 

 remain cuticular and uncalcified ; but, as growth proceeds and weight 

 increases, the larva swims less and less freely, and takes to gliding along 



