288 



ECUIXODERMATA. 



every case the arms bear, either on their main stems or on their 

 branches, lateral appendages, the 2>innules, which have an alternate 

 arrangement on each side, one being attached to each segment of the 

 arms. Essentially the pinnules represent the ultimate ramifications 

 of the arms. 



The mouth, as a rule, lies in the centre of the cup. From it 

 certain furrows, the ambulacral grooves, traverse the disc (fig. 231) 



Fiii. 233. Developmental stages of Comatul* (Ante-Ion), much enlarged, a, free-swimminq 

 larva with tuft and rings of cilia (Wr), also with rudimentary calcareous plates. 6, At- 

 tached fentacrinoid form of the same animal. O, Oralia ; R, Radialia ; B, Basalia ; 

 Cd, Centrodorsal plato. c, Older stage described as Pentacrinui airopaeus with arms and 

 cirri (after Thomson). 



and pass on to the arms, and their branches and pinnules ; they 

 are lined by soft skin, and carry the tentacle-like ambulacral 

 appendages. The anus, when it is present, lies excentrically on the 

 ambulacral (ventral) surface of the di.sc. The development of the 

 living genus Comatula, which begins with a barrel-shaped larva 

 with four bands of cilia and leads to the fixed stage of the Pen- 



