570 



CRINOIDEA 



Fig. 1. Pentacrinus. 



200) by branching; they usually bear lateral pin- 

 miles, which produce a feathered appearance. The 

 water- vascular ring does not communicate directly 



with the ex- 

 terior, but is 

 in connection 

 with the 

 body-cavity. 

 The radial 

 water -vessels 

 give off lateral 

 groups of 

 ' tube - feet, ' 

 which have 

 the form of 

 tentacles, and 

 are respira- 

 tory in func- 

 tion. There 

 is a double 

 nervous sys- 

 t e m, the 

 upper system 

 accompany- 

 ing the water- 

 vessel being 

 wholly motor; 

 the lower 

 system, with 



a central mass and radial extensions, being both 

 sensory and motor. The sexes are separate and 

 similar. The ciliated larval form (known in the 

 case of Antedon ) is not as divergent as that of the 

 other classes of Echinoderms. 



Mode of Life. The Crinoids are typically stalked 

 and more or less fixed, but the modern family of 

 Cornatulidse break off from their stem at a certain 

 stage. The stem of 

 the stalked forms is 

 usually anchored to a 

 rock or other foreign 

 object, but in some 

 cases the animal grows 

 more loosely on a 

 muddy bottom. Like 

 the extinct forms, the 

 modern stalked Crin- 

 oids grow in masses, 

 forming beds of sea- 

 lilies on the ocean 

 floor. Over a hundred 

 specimens have been 

 dredged in a single 

 haul. When they 

 move about, as in 

 the Comatulidae, they 

 do so by alternately 

 bending arid straight- 

 ening the arms. The 

 food, which consists of 

 minute organisms ( pro- 

 tozoa, diatoms, small 

 Crustacea, &c.), is 

 wafted down ciliated 

 grooves on the pin- 

 nules and arms to the 

 mouth. The known 

 larv.-e are actively free- 

 swimming by means of cilia. Comatula and some 

 others are the hosts of curious parasitic Chsetopod 

 worms (Myzostomidee), which cause deformities on 

 the arms. Like many other Echinoderms, the 

 Crinoids have the power of regenerating lost parts. 

 Broken-off arms may be replaced, and even the 

 renewal of the visceral mass has been observed. 



Important Forms and Classification. The best- 

 known genus is the cosmopolitan Antedon or Coma- 

 tula, of which A. rosacea is the most abundant 



Fig. 2. 



A, free-swimming larva, with 

 skeleton of adult forming in- 

 side; B, fixed stalked young 

 (Pentacrinus-like) stage. 



species, frequently dredged off British coasts. 

 Along with the large genus Actinometra and some 

 others less important, Antedon represents the family 

 of Comatulids, which differ from other Crinoids in 

 being stalked only in their youth. Professor J. 

 Vaughan Thompson has the credit of having dis- 

 covered the young stalked stage of A. rosacea, and 

 thus revealing the existence of a most interesting 

 habit. The free Antedon swims gracefully by bend- 

 ing its ten arms, and then settles down again for 



Fig. 3. Free Adult, showing wreath of anchoring hooks. 



a while, anchoring itself by means of an inferior 

 wreath of clawed attaching processes. Among 

 permanently stalked Crinoids may be mentioned 

 the genus Pentacrinus (e.g. P. axterius), from the 

 Atlantic and Pacific, often at great depths ; Rhizo- 

 crinus (e.g. B. lofotensis), from the Atlantic, 

 usually abyssal ; Hoi opus, from the Caribbean Sea, 

 with very massive arms, and only a single extant 

 species. These and others, being of more recent 

 origin, are distinguished as Neocrinoidea, from the 

 wholly extinct Palaeozoic forms, the Palseocrinoidea 

 or Tesselata. In the latter the cup does not in- 

 variably show that fivefold symmetry which is 

 constant in the former. 



Distribution. Though recent deep-sea explora- 

 tions have greatly increased the catalogue of living 

 forms to a total of over 400 species, distributed in 

 twelve genera, these nevertheless represent but a 

 small fraction of the extinct Crinoid fauna. Over 

 200 genera and 1500 species are known from fossil- 

 iferous strata. These begin to appear in the Cam- 

 brian, and attain maximum development in Silu- 

 rian, Devonian, and Carboniferous times. Recent 

 Crinoids never attain the size exhibited by some of 

 the fossil forms. Extinct species have been found 

 with stems 50 and 70 feet in length. 



Many of the stalked Crinoids are found at great 

 depths, and some of the Challenger dredgings 

 revealed the continuance in the deep sea of types 

 which had previously been regarded as extinct. 

 Thus the pear-encrinites (Apiocrinidae) are con- 

 tinued in the abyssal genera Rhizocrinus, Bathy- 

 crinus, and Hyocrinus. Bathycrimis gracilis has 

 been dredged from a depth of 2435 fathoms, an 

 Antedon from 2900 fathoms, Holopus and Hyocrinus 

 from 1200 to 2500 fathoms. The majority, how- 

 ever, and especially the free forms, frequent 

 shallower water. Antedon is a cosmopolitan 

 genus ; Holopus is confined to the Caribbean Sea ; 

 stalked Crinoids are found between the parallels of 

 68 N. and 46 S. lat. ; Cornatulidse between 81 

 N. and 52 S. lat. 



delations. There seems little doubt that the 

 Crinoids, Ophiuroids, and Asteroids are more closely 

 linked to one another than to Echinoids and Holo- 

 thurians. In some ways the Crinoid type is struc- 



