2 'J 



DK. J. F. GEMMILL OX 



the Orders Plianerozonia and Cryptozonia, these Order's being 

 called by Sladen, in consequence, the Stenopneusia and Adeto- 

 pneusia respectively. 



That the division in question is not an altogether natural one 

 has been pointed out by various authorities (e. g. Jeffrey Bell, 1, 

 and MaeBride, 4). Further, as I have recently proved (2), the 

 larval history of the phanerozonate Porania resembles in its 

 essentials that of the typically cryptozonate Asterias rubens L. (3), 

 both species having a feeding bipinnarial larva which changes 

 into a brachiolaria and becomes attached at metamorphosis. Pro- 

 bably the occasional presence of infra-marginal gills in Porania is 



Text-figure 1. 



Porania pulvillus. 



Vertical section through interradial marginal edge, showing 

 the abnormal distribution of gills. 



ah.b.w., abactinal body-wall; ab.ff., abactinal gills; act.b.w., actinal body-wall; 

 act.ff., actinal gill ; pi., part of a marginal plate ; mg., marginal edge, with 

 spine. 



not due directly to atavistic or ancestral causes, but is a parallel 

 manifestation, in an individual belonging to a particular asterid 

 Family, of a tendency or potency which has been fully realised 

 in the various members of numerous other Families. 



In any case, the specimen of Porania here described* deserves 

 notice, because, although, as is well known, the Linckiida? (6, 

 p. 397) include genera some of which have, and others have not, 

 actinal gills, no instance of abnormal gill-distribution within a 

 particular phanerozonate species appears to have hitherto been 

 recorded. 



* Three other specimens of Porania with actinal gills have recently been obtained 

 at Millport. Several such gills are present in the specimen illustrated in fig. 1, 

 PI. I. of the preceding paper. 



