576 THE ANIMAL KINGDOM. 



parasitic order of Chaetopoda ; see p. 609. Extinct species seem to have 

 been similarly attacked (von Graff). 



Holopus and Hyocrinus are abyssal forms (1200-2500 fathoms); an 

 Antedon has been dredged at 2900 fathoms, but the majority of Crinoidea 

 occur in shallower waters. They are generally met with in masses, as is the 

 case with Crinoids in the Silurian, Carboniferous, and Oolitic rocks. Antedon 

 is a cosmopolitan genus ; Holopus is confined to the Caribbaean sea. 

 Stalked Crinoids are found between the parallels of 68 N. and 46 S. lati- 

 tude ; Comatulidae between 81 N. and 52 S. The Holopodidae originate in 

 the Upper Silurian, Pentacrinus in the Trias, Antedon in the lower Oolite. 

 Many fossil Comatiilidae are large in size. 



The gastrula mouth in Antedon closes early, and is not terminal and posterior, 

 as in other Echinodermata, but behind the larval third ring of cilia, on a flat 

 ventral surface (cf. p. 548, ante). The water- vascular vesicle is formed after the 

 left and right coelomic vesicles, which become placed terminally, the left orally, 

 the right aborally at the base of the stem into which it extends. The water- 

 vascular vesicle grows round the oral end of the archenteron, and subsequently 

 divides the left peritoneal vesicle into an anterior and posterior portion. The 

 former forms the oral vestibule, enclosed within the oral plates. The mouth is 

 formed in its floor, and the ambulacral grooves grow out from it radially. Hence 

 their epithelium is hypoblastic in origin. The posterior section sends out the 

 subtentacular canals, as the right vesicle does the coeliac. The water-vascular ring 

 developes ten pairs of tentacles within the vestibule. Five pairs are simple and 

 short, and are interradial like the oral plates. The other five are radial, and consist 

 each of three tentacles, the central one of which is carried outwards in the growth 

 of the arm. The five orals are at first large, and one is pierced by the primary 

 water-pore ; they are eventually resorbed as is the anal interradial plate, which at 

 first touches a basal, and is afterwards raised on to the disc. 



The Crinoidea are divisible into 



1. Palaeocrinoidea (= Tesselatd) : calyx not invariably pentamerous ; ategmen 

 calycis generally present ; anal interradius large, with an anal interradial touching a 

 basal. Palaeozoic. 



2. Neocrinoidea (= Articulata + Holopus + Marsupites] : calyx pentamerous ; 

 interradials rarely intervene between the radials ; orocentral always absent ; orals, 

 when present, surround the mouth. Mesozoic and recent. Comatulidae and other 

 living Crinoidea. 



General anatomy, P. H. Carpenter, Challenger Reports, xi. 1884. New sp. of 

 Metacrinus, Id. Tr. L. S. (2), ii. (14), 1885. Variations in structure of arms, Id. 

 ibid. ii. (17), 1886. 



Fossil forms. See in addition to Zittel's work, cited p. 549, ante, de Loriol, 

 4 Palaeontologie Francaise,' Paris, x., pt. i, 1882-84; Wachsmuth and Springer, 

 'Revision of Palaeocrinoidea] Proc. Acad. Nat. Sc. Philadelphia, 1879, 1881, 1885 ; 

 cf. P. H. Carpenter, supra, and in A. N. H. (5), xvii. 1886, p. 276. 



Deformities in fossil Crinoids due to Myzostomidae, von Graff, Palaeonto- 

 graphica, xxxi. 



