STRUCTURE OF CRINOIDS. 217 



its normal position, in the others it is at the anchoring base 

 of the stalk. When Comatulids break off from their larval 

 stalk they carry with them the topmost joint, which becomes 

 the central part of their calyx. 



The oral disc, turned upwards, is supported by plates. 

 Here the anus also is situated. The arms usually branch in 

 dichotomous fashion, and thus ten, twenty, or more may 

 arise from the original five. But the growing point continues 

 to fork dichotomouslyj like the leaf of many ferns, and as each 

 alternate fork remains short, a double series of lateral 

 " pinnules " results. The arms are supported by calcareous 

 plates. The stalk usually consists of numerous joints, 

 especially in extinct forms, in some of which it measured fifty 

 and even seventy feet in length. Except in Holopus, and in 

 the stalked stage of Antedon, the stalk bears lateral cirri. 



The nervous system is remarkable, in being double. On 

 the upturned surface of each arm, beneath the food-wafting 

 ciliated grooves, there is a motor ambulacral or radial nerve. 

 These are united in a ring or plexus around the mouth. So 

 far, the Crinoid is like a starfish. But on the opposite 

 surface, there is an antambulacral motor and sensory nervous 

 system consisting of a central mass, with branches to the 

 cirri, and to the arms. Apart from the superficial epithelium 

 there are no sensory structures. 



The ciliated food-canal descends from the mouth into the 

 cup, and curves up again to the anus which is usually ex-centric 

 in position. From the cup, where the body-cavity is in great 

 part filled with connective tissue and organs, two coelomic 

 canals extend into each of the arms. One of these — called 

 subtentacular — lies just beneath the radial water-vessel ; 

 the other — called coeliac — is more internal, but still ventral. 

 They communicate at the apices of the arms and pinnules, and 

 currents pass up the subtentacular and down the coeliac canal. 



The blood-vascular system consists of a circumoral ring 

 which is connected with a radial vessel under each ambulacral 

 nerve, and with a circumoesophageal plexus. There is also a 

 " plexiform organ," " lying interradially in the disc anteriorly 

 to the mouth," with many connections with the above- 

 mentioned plexus, with the vessels to the organs, and with 

 a strange " chambered organ " which lies within the central 

 aboral nervous system, and gives off vessels to the stalk and 



