168 



PHYLUM ECHINODEKMATA. 



the abactinal surface of the disc in interradius I. II, very nearly, 

 but not quite, at the central point (Fig. 83). 



The flattening of the body is carried furthest in Palmipes 

 membranaceus, which has the form of a pentagonal sheet of card- 

 board. While the oral surface is generally flat, the aboral sur- 

 face is often arched, sometimes considerably so (Pteraster, 

 Hymenaster, Marginaster, Pentaceros, Culcita, many Solasteridae 



FIQ. 118.Echinastersentus from the oral surface (after A. Agassiz). mouth. 



A f ambulacra! feet. 



and Asteriidae). The rays are usually approximately equal in 

 size ; inequality generally implies recent mutilation and re- 

 generation. 



Starfishes vary in size from those with an arm-radius (centre 

 of disc to extremity of ray), of 1 cm. or less (Marginaster 

 pentagonus, '3 cm.) to those in which the same dimension 

 measures 45 centimetres (Luidia savignyi to 37 cm., Freyella 

 remex to 45 cm.). 



