288 



ECHINODERMATA. 



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

 branches, lateral appendages, the pinnules, 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) 







FIG. 233. Developmental stages of Comntnla (Anfedon), much enlarged, a, free-swimming 

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

 tached Pentacrinoid form of the same animal. O, Oralia ; S, Radialia ; S, Basalia ; 

 Cd, Centrodorsal plate, c, Older stage described as Pentad-inns europaeus 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 disc. 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- 



