MONOGKAPH OF THE EXISTING CRINOIDS. Ill 



tube, is known as the anal area, and it is from this area that all crmoids are 

 oriented, a plane passing through the anal tube and tlirough the mouth, and there- 

 fore also along the ambulacrum leading to the anterior ann, and through the center 

 of the so-caUed anterior radial and anterior post-radial scries of ossicles (division 

 series and free undivided arms), dividing the animal into two equal halves, which 

 exhibit more or less, in the Comasteridje often very pronounced, bilateral symmetry 

 (seefigs. 22-2S, p. 69). 



In certam species of the Comasteridse the mouth moves from the original 

 position at the base of the anterior ray to a position between tiie bases of the 

 anterior and the right anterior rays; this results in making the left branch of the 

 peripheral ambulacral furrow much longer than the right branch; a balance between 

 the two is attained by the d\vuidlmg and eventual suppression of that part of the 

 left branch which supplies the left posterior ray, so that the two maui ambulacral 

 furrows are agam equal, each supplying two arms or rays, the fifth ray being quite 

 devoid of ambulacra. Tliis fifth ray, after the loss of its ambulacra, becomes much 

 reduced m size. There is now a well-marked bilateral symmetry, quite different 

 from the origuial bilateral symmetry; a plane passmg through the center of the 

 left posterior arms and division series and along the center of the left posterior 

 IBr series and radial, thence tlu-ough the (central) anal tube and interradial mouth 

 situated between tlie bases of the anterior and right anterior rays, divides the 

 animal uito two equivalent halves. The plane of bilateral sjTiimetry has therefore 

 become slufted, in the direction of the movement of the hands of the clock, one- 

 tenth of a circumference, or 36° (see figs. 27, 28, p. 69). 



In the Comasteridse and Uintacrinidte the anal area is usually of very much 

 greater size than any of the other interambulacral areas, including sometimes 

 almost the entire surface of the disk and forcing the ambulacral grooves and mouth 

 to a marginal position. The anal tube in these two families is usually nearly or 

 quite at the center of the disk, whereas in the other families it is marginal or 

 submarginal (see figs. 22-28, p. 69). 



Set closely together in a single Ime along each side of the ambulacral grooves 

 of the disk, arms and pinnules (except in the species of the family Comasteridse) 

 are small round bodies, usually (in preserved specimens) yellow, or various shades 

 of red and violet to nearly black in color (though colorless ui life), known as Sacculi 

 (see fifs. 15, 16, p. 67). These sacculi are of some hnportance systematically, 

 varymg greatly in abundance and in distribution in different groups. 



In certain species of the Comasterida; there are found upon the posterior un- 

 grooved arms much larger rounded bodies knowm variously as Spherodes, Ovoid 

 bodies or Sense organs. 



The perisonie of the disk alwaj's contains in its inner layers calcareous concre- 

 tions of secondary (p'erisomic) origin. These often become much enlarged and 

 thickened so as to project above the surface of the disk m the form of prominent 

 calcareous nodules which may be scattered or, if they are very numerous, may 

 form a solid calcareous pavement, in which case the disk is said to be Plated (see 

 ficrs. 18, 19, p. 67). These nodules or plates are most commoidy found in the 

 anal area about the base of, or on, the anal tube, or m the interprunibrachid areas, 



