Characteristics of Crinoidea. civ 



along the furrows, which radiating" from the mouth pass down 

 the medio-ventral surface of the arms, and of their laterally arti- 

 culated pinnulae. But the water- vascular system appears to be in 

 the Crinoidea wholly, as it is in other Echinodermata partly, 

 respiratory in function. The segmented arms can execute exceed- 

 ingly active movements by means of the muscles with which 

 they are provided ; but these movements are mainly confined to 

 the action of closing the arms upon the oral surface of the disc, 

 and so protecting it from the contact of irritating matters. The 

 act of swimming from place to place however by the alternate 

 action of the arms, is by no means so habitual to the Crinoidea as 

 has been supposed. The Crinoidea are dependent mainly npon the 

 action of cilia lining their digestive tract, and partly upon that of 

 the similar structures npon the integument of their arms, for the 

 ingestion of alimentary matters. Their oral surface is, in the 

 natural condition, always turned upwards ; the digestive tract, 

 in the living genera, Antedon and Pentacrinus, possesses an anus 

 opening in one of the spaces between two of the water-vascular 

 furrows radiating from the mouth. The digestive system is confined 

 to the central disc ; but the perivisceral space has tubular prolong- 

 ations along the arms and their pinnules, whereby the nutritive 

 fluid is on the one hand itself aerated, and on the other brought into 

 relation with the powerful muscles whereby the movements of the 

 arms are executed. Their generative organs differ from those of 

 all other Echinodermata in being lodged externally in the mem- 

 branous lamellae developed from the ventral surface of the pinnulae, 

 and in setting free their products simply by dehiscence. The larva 

 or pseud-embryo gives rise within itself to a second form, which, 

 without taking up any jjart of the short digestive tract of the first, 

 developes a peduncle which is discarded in Antedon, but is per- 

 manently retained in Pentacrinus. 



Jointed cirri project at intervals from this peduncle in Pentacrinus, 

 and in the non-pedunculate Antedon are attached to the aboral surface 

 of the calyx. The existence of these cirri would appear to show that 

 the true homologiie of the 'arms' of the Crinoidea is to be found in 

 the ambulacral tentacles of the Holothurioidea, whilst they themselves 

 are homologous with the radial ambulaci'a of other Ecliinodci-mata. 

 It shows further that the Echinodermata are in Mr. Herbert Spencer's 

 language 'tertiary' rather than 'secondary aggregates,' a view to which 



