394 ARTHUR T. MASTERMAN. 



papilla has increased in size. The ridge now passes to the 

 latest developed pair, and has a prominent bay at the spot at 

 each corner of the hood where the atrial grooves emerge. 

 The tentacles may be regarded as arising on this ridge one 

 by one. If the ridge prove in future investigations to be 

 ciliated, the early Actiuotrocha at this stage would have a 

 single post-oral ciliated band comparable to that of Bipin- 

 naria, which in later life breaks up like that of this Echino- 

 derm into a pre oral and a post-oral band. 



Under any circumstances the pre-oral band edging the 

 pre-oral hood, and the post-oral band following the course of 

 the tentacles, are connected at these early stages by a thick- 

 ened ridge which probably indicates a phyletic unit3^ The 

 common origin of these two bands from one " architroch " 

 has been suggested by Lankester (9a). 



In fig. 18 the tentacles spread out in such a way as to 

 hide the anal process altogether, but it may still be recog- 

 nised by reversing the larva. 



Fig. 19 shows a front view of the same stage ; the large 

 bell-shaped pre-oral lobe is conspicuous, and it extends out 

 laterally beyond the body. This is the latest stage found in 

 the tentacles of the parent, and it is probable that rupture of 

 the egg-membrane is effected after its attainment, the larva 

 then leaving the lophophore of the parent. 



The internal structure of the stage with two pairs of ten- 

 tacles is intermediate in character between that with one 

 pair which has been described, and that with three pairs 

 (figs. 18 and 19). The internal structure of this latter stage 

 is indicated by the sections shown in figs. 46, 47, 48, and 49, 

 and by the restoration in fig. 50. 



There is little change in the epiblast, except that the central 

 nerve- ganglion {ng.) can be discerned in figs. 46 and 49 as 

 a thickening of the epiblastic cells over the oesophagus. The 

 stomodseum (oesophagus) is now very long and curved round 

 into the stomach, which is still simple, and does not appear 

 to have yet given rise to the pleurochords. The hind end of 

 the alimentary canal is constricted off to form the intestine 



