408 • DR ARTHUR T. MASTERMAN ON THE 



be indicated as a deflection of the body upon the pre-oral lobe over towards the right- 

 hand side of the larva and back again. As a matter of fact, Goto maintains that in 

 Asterina he has recognised just such a dextral bending, which is, however, largely dis- 

 guised by the sinistral bending which follows it, and so eventually brings the oral surface 

 down to the substratum. This ' rocking ' of the ' body ' on its stalk and pre-oral lobe 

 has no other meaning than the vestige of a more complete rotation through 180° and 

 backwards. In addition, it is a common phenomenon in pelagic larvas for the disc to 

 be at first directed with its oral face posteriorly. 



Without considering other points of view, I think this is to be expected from the 

 conditions of existence in the two orders. Diagram 4 will illustrate the point. In A the 

 fixed asterid breaks away from its stalk, turns over through 180°, and takes up its 

 position on the sea-floor. If this process takes place earlier in development (B), the 

 asterid never looks upwards, but drops off obliquely ; and if it is still further hastened 

 (C), the pre-oral lobe never reaches the aboral surface, but is first lateral, and then is 

 bent orally, as actually takes place. Antedon, with its longer period of sedentary life 

 (phyletically), has not shortened matters to this extent. This may not be the true 

 explanation, and MacBride may be perfectly correct in his inference, but such an ex- 

 planation should at least be borne in mind. 



The principal organs affected by the axial symmetry in stages E and F are the 

 radiate hydroccele, the disc-shaped epigastric ccelom, the annular hypogastric ccelom, the 

 pentamerous perihsemal and oral rudiments. 



So far, therefore, as we can read phylogeny into ontogeny, we may find in the 

 embryonic stages of Cribrella the well-marked ccelenterate in the gastrula ; in stages A 

 and B of the larval period we have the ccelenterate with coelomic pouches, very nearly 

 conforming to a tetramerous axial symmetry, and comparable to those in the larva of 

 Balanoglossus. In stage C the bilateral archi- ccelom ate asserts itself, embryonically 

 sketched. This culminates in stage D, in which there are already forcible indications of 

 the creeping habit, with its sinistral asymmetry. We may recall the actual resting 

 habit of the living larvae at this stage. Fixation next follows, and in stages E and F 

 the axial symmetry rapidly appears. The disappearance of the median sagittal mesen- 

 tery and the radiation of the hydroccele lead the way, and the other processes follow in 

 rapid succession. 



It should be noted that, on the interpretation here shown, the adult circular mesen- 

 tery is not a median mesentery of the bilateral stage, but is a primary mesentery, 

 between the posterior and right lateral cceloms. 



Comparison with other Groups of Echinoderms. 



With present knowledge it is perhaps premature to attempt a comparison in a de- 

 tailed degree with other groups, but there are a few points to which some reference 

 should be made. 



