ON ABNORMAL GILLS IN A STARFISH. 21 



2. Abnormal Gills in the Starfish Porania pidvillus 0. F. M. 

 By James F. G-emmill, M.A., M.D., D.Sc, F.Z.S. 



[Received- October 1, 1914 : Read February 23, 1915.] 

 (Text-figure 1 .) 



The genus Porania belongs to the Gymnasteriicla?, a family of 

 the Phanerozonia (Sladen, 6, pp. xxxiii, 360). In P. pidvillus 

 the disc is large and the arms short, and the actinal and actinal- 

 intermediate areas are flattened, so that they rest more or less 

 closely against any surface to which the starfish is adherent. The 

 margin or boundary between the actinal and abactinal areas forms 

 a sharp angle and is exceptionally well defined owing to (a) the 

 contrast in colour between the white actinal and the crimson 

 abactinal surfaces, and (6) the presence of spines which project 

 horizontally outwards from the margin and are set on the 

 marginal plates. The actinal intermediate plates are very regular 

 and are arranged in short rows, standing out more or less at right 

 angles to the rays, but sloping slightly towards the mouth. Super- 

 ficially on the actinal epiderm, ciliated grooves overlie the inter- 

 spaces between the rows of plates. On the abactinal surface near 

 the margin, the papula? or gills come close down to the supero- 

 marginal plates. 



Apart from the superficial grooves above mentioned, the actinal 

 intermediate areas, as a rule, are smooth and destitute of gills, 

 spines, or other growths, except along the margins of the 

 ambulacra, which are bordered by double or triple rows of spines. 

 However, in a preserved specimen from the Millport Marine 

 Station, which 1 examined for other purposes, there appeared on 

 the actinal surface a set of opaque papilla? arranged with no little 

 regularity, one in each of the grooves above mentioned, a short 

 distance inwards from (*. e. to the oral side of) the margin. These 

 papilla?, being soft, were obviously not spinous in character, but, 

 owing to the thickness and opacity of the body-wall, the question 

 whether they were gills could not be satisfactorily decided by 

 inspection, even after the coelomic cavity was laid open and viewed 

 from inside. Serial sections were accordingly made, and the 

 structures referred to were then found to be perfectly typical 

 papula? or gills. The accompanying figure illustrates somewhat 

 diagrammatically a section, vertical to the marginal edge, show- 

 ing one of the infra-marginal and three of the supra-marginal 

 gills. 



As Joh. Midler first noted (5, p. 163) and as was emphasised 

 later by Sladen (6, p. xxiv), starfish which possess well-developed 

 marginal plates have their papula? or gills limited to that part of 

 the abactinal surface which is bounded by the supero-marginal 

 plates. This provides one of the important distinctions between 



