244 ECHINODERRIA. 



Stalk. The modern Comatulids, e.g., the rosy feather star 

 [Comatula or Antedon rosaceus) leave their stalk at a certain 

 stage in life ; but the other Crinoids, e.g., Pentacrinus, are 

 permanently stalked like almost all the extinct stone lilies or 

 encrinites once so abundant. Most of them live in deep 

 water, and many in the great abysses. An anchorage is 

 found on rocks and stones, or in the soft mud, and great 

 numbers grow together — a bed of sea lilies. The free 

 Comatulids swim gracefully by bending and straightening 

 their arms, and they have grappling "cirri" on the aboral 

 side, where the relinquished stalk was attached. By these 

 cirri they moor themselves temporarily. Small organisms — 

 Diatoms, Protozoa, minute Crustaceans — are wafted down 

 ciliated grooves on the arms to the central mouth, which is 

 of course on the upturned surface. Some members of the 

 class, e.g.,Comatula, are infested by minute parasitic "worms" 

 (Myzostomida;) allied to Chajtopods, which form galls on the 

 arms. A lost arm can be replaced, and even the visceral 

 mass may be regenerated completely within a few weeks 

 after it has been lost. 



The animal consists of (i) a cup or calyx, (2) an oral disc forming the 

 lid of this cup, (3) the radiating " arms," and (4) the stalk supporting the 

 whole. 



The calyx consists of the topmost segment of the stalk, a centro-dorsal 

 plate, and several rows of radial plates, which lead on to the brachial 

 plates of the arms. When Comatulids break off from their larval stalk, 

 they carry with them the centro-dorsal plate, (CD. in Fig. 79), which 

 becomes the central part of their calyx, and bears the cirri. 



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 dichotomously, 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 over fifty feet in length. Except in Holopiis, 

 and in the stalked stage of Aiilcdon, 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 subepithelial nervous band, probably in great part sensory (Fig. 79, h). 

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

 the Crinoid is like a starfish. But on the dorsal surface the main mass 

 lies — an antambulacral motor and sensory nervous system, consisting of a 

 central capsule, with branches to the cirri and to the arms (d and a in 

 Fig- 79)- Cuenot asserts that this system, though to a very slight extent, 



