384 DR ARTHUR T. MASTERMAN ON THE 



the emancipated larvae. One can hardly suppose that the mother, in natural surround- 

 ings, allows her offspring to make excursions out of the brood-chamber like chickens out 

 of a hen-coop, and there is no indication of this habit in mothers kept under observation. 

 On the other hand, it is quite possible that a certain proportion of the mothers perma- 

 nently set free their offspring during the larval stages, in which cases there is every 

 reason to suppose that normal development would proceed, though a higher mortality 

 may be incurred. In any case Cribrella is evidently, from an evolutional point of view, 

 just acquiring the habit of brooding, and at present the later larvae certainly appear 

 more suitably equipped for free life than for quiescent existence within the brood-chamber. 

 Probably the ciliary action is still of great functional importance in the aeration of the 

 brood-chamber. 



After stage D, the free life (in aquaria) comes to an end. Fixation is effected, as 

 in fig. 48, by the pre-oral processes, and indications of the pentamerous axial symmetry 

 commence to appear. It must be said that a large number do not fix themselves, but 

 lie on their right side ; these do not appear to be detrimentally affected, but pass on to 

 later stages without any obvious abnormality. 



Before leaving stage D, it may be observed that many of the larvae show indica- 

 tions of a division of the dorsal pre-oral process into two (fig. 47). This is interesting 

 when we notice that the larva of Asterias miilleri at a corresponding stage has the 

 dorsal process completely divided into two, so that the anterior end of the larva bears 

 four equal tentacle-like processes radiating to the four points of the compass. Asterina 

 (with two processes), Cribrella (with three), and Asterias miilleri (with four) thus present 

 a continuous series in the evolution of the pre-oral lobe. At no stage in the larval 

 period is there a mouth or anus, and the first opening to appear is the water-pore, which 

 is recognisable early in stage E (sometimes in D) as a small aperture in the centre of 

 the right side. We may correlate the absence of any ciliated bands with the absence 

 of mouth and with the presence of yolk. At no period does the young Cribrella depend 

 upon the outside world for food till it is adolescent, hence the nutritive bands of the 

 Bipinnaria are not required, and the powerful motor bands of Antedon larva are equally 

 useless. The yolk is, as has been seen, in the last embryonic stage, evidently scattered 

 throughout the cells in the form of granules of varying size, enabling development to 

 proceed independently of the capture or acquirement of food. But although this is the case, 

 there is to be discerned no essential difference in the course of development which would 

 justify the application of a term like 'direct' development, the term 'indirect' being 

 similarly applied to the process in Bipinnaria. I have carefully studied both processes, 

 and confess that the distinction thus emphasised does not appear to be valid. The 

 presence of lecithal nutrition, like that of hsemal nutrition in viviparous forms, no 

 doubt accounts for certain marked adaptations not unlike those characteristic of 

 parasites, such as loss of alimentary, sensory and motor organs, but there can be little 

 question that the larval ' entity ' is evident both in time and space in these demersal 

 types equally with the pelagic. In the former the pre-oral lobe becomes specialised for 



