470 



ECHINODERMATA ASTEROIDEA 



CHAI 



the interspaces between the marginal plates, these also subserve 



respiration by 

 drawing a current 

 of water over the 

 gills. Psilaster 

 (Fig. 202). 



Fam. 3. Por- 

 cellanasteridae. 

 Paxillosa which 

 have lost the anus. 

 There is a conical 

 prominence in the 

 centre of the dor- 

 sal surface termed 

 the epiproctal 

 cone, and in the 

 interradial angles 

 there are vertical 

 grooves bordered 

 by folds of mem- 

 brane produced 

 into papillae, the 

 so-called " cribri- 

 form organs." The 

 marginal plates 

 are thin and form 

 the vertical border 

 of the thick disc. 

 The tube - feet 

 have no suckers. 



Comparing the 

 Porcellanasteridae 

 with the Astro - 

 pectinidae we see 

 at once that the 

 " epiproctal cone n 

 is a permanent 

 representative of the temporary aboral elevation in Astropecten, and 

 we are inclined to suspect that the cribriform organs are grooves 

 lined with cilia which keep up a respiratory current like the ciliated 



Fig. 203. 



-Porcellanaster caeruleus. A, aboral view ; B, oral 

 view, x 1. (From Wyville Thomson.) 





