400 



DR ARTHUR T. MASTERMAN ON THE 



side of the larva, the aboral from the right, and no amount of twisting of the degener- 

 ating pre-oral lobe at a later stage can nullify this fact. 



Kecollecting that the sagittal larval plane is parallel to the oral and aboral adult 

 planes, the larval left corresponding to the former and the larval right to the latter, we 

 may go on to inquire the further relation of the madreporic plane of the adult. By 

 this is understood a plane passing through the axis of symmetry, through the madre- 

 poric inter-radius and through the opposite radius. This is often referred to as the 

 plane of bilateral s} 7 mmetry. I have numbered the hydroccele radii in the order of 

 their appearance, the antero-ventral being 1 , the most posterior being 3, and the antero- 

 dorsal being 5. This is the reverse order to MacBride's in Asterina, but naturally 

 No. 3 corresponds in each case : it agrees with the order adopted by Ludwig* and 

 GoTO.f The adult bilateral plane passes through 4/5 inter-radius across to the 

 postero-ventral radius, i.e. No. 2. This will be clear from a consideration of the 

 following : — In stage D the figures 50, 52 and 57 all show clearly that the pre-oral 



2. — Diagram of Stages D, E and F of Gribrella larvse seen from the left side, illustrating the changes 

 in the posterior ccelom and its relationships to the hydrocoele and pre-oral ccelom. (A) Stage D, 

 showing the posterior ccelom extending in a curve from 4/5 to 1 (or about 250°). (B) Stage E, show- 

 ing the posterior ccelom extending from 4/5 to 5/1 (or about 290°) ; all the extension is in the ventral 

 arm, the dorsal remaining still in 4/5. (C) Stage F, showing the posterior ccelom now extending 

 through about 340°, a small canal of pre-oral ccelom alone separating the two ends ; as before, all the 

 growth has been effected by the ventral arm. The dorsal arm gives off an aboral rudiment (V.) over 

 the hydroccelic radius 4. The next (IV. ) is formed at the posterior end over 3, and the other three, 

 III., II., and I., are formed by the ventral arm over 2, 1 and 5 respectively ; hence an unconforma- 

 bility is produced, but there is no torsion. 



ccelom has a posterior wall abutting against the dorsal horn of the left posterior 

 ccelom. Eventually, in stage F the latter acquires an aperture into the former, but 

 the relation of these two never changes. Along this posterior wall of the pre-oral 

 ccelom is formed the stone-canal, which passes into the pore-canal on the right and into 

 the hydroccele on the left (cf. fig. 63). Fig. 64 shows that immediately the hydrocoele 

 becomes radiate, the stone-canal opens into it in the 4/5 inter-radius, and the relationships 

 of these parts remain the same throughout. On the other hand, the left posterior 

 ccelom lies round in a crescent with its ventral horn pressing (in figs. 55 and 52) against the 



* Ludwig, H., " Entwicklungsgeschichte der Asterina gibbosa," Zeitsch. f. w. Zool., vol. xxxvii., 1882. 

 + Goto, S., Journal of the College of Science, Imp. Univ. Tokyo, Japan, vol. x. part iii., 1898. 



