CB1NOIDEA. 



289 



tacriiius form (P. Europceus) (fig. 233), consists of a complicated 

 metamorphosis. 



The greater number of Crinoids belong to the oldest periods of 

 the history of the earth (the Cambrian, Silurian, Devonian, and 

 the Carboniferous formations). Existing forms 

 live mostly at considerable depths. 



We distinguish two orders, the Tesselata and 

 the Articulata. 



The latter is represented by numerous fossil 

 forms, but by only a few living genera as Penta- 

 crinus, Holopus, and Comatula (fig. 234). The 

 cup is always less completely provided with plates 

 than in the fossil Tesselata. 



ARTICULATA. 



Fam. Pentacrinidae. Crinoids with ten arms, several 

 times bifurcated. There is a pentagonal stalk with 

 whorled cirri. Pentacrinus caput Medusa, Mill, from 

 the Antilles. P. Miilleri Oerst, West Indian Ocean. 

 The fossil forms are : Encrinus liliiformis Schl. (fig. 234) 

 from the Muschelkalk ; also Apiocrinus, allied to the 

 existing Rliizocrinus lofotensis Sars, and to Satliycrinus 

 gracilis, and aldricliianus W. Th., from the deep sea. 

 Allied to this group is the third existing genus Holopus, 

 from the West Indies, with calyx attached by a short Flo< 234. _ Encnmis lU'd- 

 unjointed prolongation of its apex. II. Rangii d'Orb. formis from the Mus- 



Fam. Comatulidae. Stalked only in the young state. chelkalk. 

 The adult animal is free. There are usually ten arms at 



the margin of the flattened body ; mouth and anus are present. The Coma- 

 tulidcB possess the power of striking their arms towards the ventral surface 

 and so of propelling themselves amidst the sea-weeds. The vermiform larva, 

 with its four ciliated girdles, makes its appearance within the egg-membranes. 

 It acquires a mouth and anus, also a tuft of cilia at the posterior end of the 

 body, and swims about freety. It passes later, by the formation of cal- 

 careous rings and rows of plates, into the stage of the stalked Pentacrinus, 

 from which the Comatula is produced by the separation of the cup from 

 the stalk. Comatula mediterranea Lam., Antcdon rosacea Link., known 

 in the young attached stage as Pentacrinus Europaeus. Actinomctra 

 J. Mull. 



To the Crinoids are allied the fossil Cystidea and Blastoidca. The Cystidea 

 were provided with short stalks and slightly developed arms. Their generative 

 organs were enclosed in the calyx, whence their products escaped through a 

 genital opening capable of being closed by movable valves. They are found as 

 fossils in the Cambrian, Silurian, and Devonian formations and the Carboni- 

 ferous limestone. To this group belong the genera Spliaeronites, Caryocrinus, 

 Apiocystitcs. 



The Blastoidea have no arms, and only possess ambulacral areas on the calyx, 

 which is attached by a segmented column. Pentatrematitcs. 



19 



