CRINOIDEA. 



289 



and is described at p. 152, takes place in the water and never in 

 brood pouches. 



Arms very readily break off so that the animals can escape 

 if they become entangled or seized by enemies ; the rupture takes 

 place at a syzygy. Arms so 

 lost are regenerated, but no 

 cases of asexual reproduction 

 have been met with. Regenera- 

 tion of the viscera takes place 

 after evisceration, and sponta- 

 neous evisceration is said some- 

 times to occur. 



The number of radii, i.e. of 

 primary arms, is nearly always 

 five, but some extinct forms 

 show indications of having had 

 a different number, e.g. Plicato- 

 crinidae with 4, 6, or 8 (rarely 5 

 or 7) radials.* Moreover speci- 

 mens of Ehizocrinus are found 

 with 4, 6, or 7 radii, and of the 

 few specimens of Holopus (Fig. 

 210; that have been found one 

 was tetramerous. Among Coma- 

 tulae also forms with 4 and 6 

 rays are very rarely met with. 

 Except in Rhizocrinus, it is rare to find the number five 

 departed from. 



The Crinoids, especially those living near the shore and to a 

 depth of 150 fathoms are gregarious in their habits, and the 

 remains of the Crinoid forests in the Silurian and Carboniferous 

 rocks show that this habit is not conPned to living forms. In 

 the Crinoid forests found in Palaeozoic rocks different genera and 

 species, belonging even to different orders, are associated, 

 whereas after Palaeozoic times the Crinoid forests consist of 

 associations of individuals of the same species. Antedon (Fig. 

 188) is free but can anchor itself by its cirri ; Rhizocrinus (Fig. 

 207) is attached by a branching root, while the lowest joint 



FIG. 198. Transverse section througii 

 a pinnule of an adult female of Ante- 

 don (after Ludwig). a sacculus ; 

 b septum between the subtentacular 

 canals ; Br perihaemal canal ; CD 

 coeliac canal ; CG genital canal ; 

 CV subtentacular canal ; d mem- 

 brane separating the coeliac canal 

 from the genital canal ; e ciliated pit 

 of the coeliac canal ; E eoithelium of 

 ambulacral groove ; I' trunk of 

 apical nervous system ; O cavity of 

 the ovary ; K calcareous plate ; 

 Nr radial nerve ; q membrane en- 

 veloping the ovary ; T tentacles 

 (tube-feet) ; Wr radial water-vascular 

 canal. 



* The living Promachocrinus is peculiar in having 10 radials. 

 Z III. U 



