ASTEROIDEA 49 



Caprellid, Podalirius typicus, is often found on Asterias rubens, 

 but is hardly to be regarded as parasitic. Another Crustacean, 

 Dendrogaster, a Cirripedian, is strictly parasitic, transformed into 

 an irregular, branched body, without limbs, full of eggs and 

 larvae. It has been found in Solaster and Henricia, but is not 

 known from British seas. A fish, Fierasfer, lives in the body 

 cavity of a large tropical sea-star, Culcita ; probably it enters 

 through the mouth of the sea-star and must then pierce its way 

 into the body cavity through the wall of the stomach. 



Fossil Asteroids are not very numerous and mostly poorly 

 preserved ; accordingly they do not play so important a role in 

 geology as do Crinoids and Echinoids. 



The classification of Asteroids is, in the main, fairly well 

 established. There is a general agreement that they are naturally 

 divided into the following main groups : PaxiUosa, Notomyota, 

 Valvata, Sjnnulosa, and Forclpulata ; it is only questionable 

 whether these may again be arranged in the two higher groups 

 introduced by Sladen, the Phanerozonia, comprising the three 

 first, and the Cryptozonia, comprising the two last. Recent 

 authorities on the Asteroids incline to the view that the three 

 first groups should be united in one order, the Phanerozonia, 

 while the two latter are regarded as orders of equal rank with 

 the Phanerozonia. This classification is adopted here. 



Regarding the question which group represents the more 

 primitive type, opinions differ. Most of the recent authorities 

 on Asteroids regard the Phanerozonia as the most primitive 

 type, but others hold that the Spinulosa represent the most 

 l^rimitive type. Palaeontological evidence is strongest for the 

 Phanerozonia as the more primitive, the oldest known Asteroids 

 {Cambraster, from Middle Cambrium ; Archegonaster, from Lower 

 Ordovician) being decidedly of phanerozoniate type. 



Key to the orders of Asteroids. 



1. Marginal plates usually large and conspicuous. Papulae mostly 



restricted to the dorsal side ; pedicellarise never of the crossed 



type ; tube-feet in two series . . I. Phanerozonia 



Marginal plates usually inconspicuous. Papulae generally also 



found on the oral side . . . . . . . 2 



2. Pedicellarise, which are very rare, never of the crossed type. 



Spines of the dorsal side usually in groups ; tube-feet usually 

 in two series ...... II. Spinulosa 



Crossed pedicellarise present. Spines of the dorsal side single or, 

 more rarely, in groups ; tube -feet in 2-4 series 



III. Forcipidata 

 E 



