2l6 MANUAL OF ZOOLOGY. 



Numerous living forms of Comatula are known, and have 

 been described under various subordinate types (Antedon, 

 Actinometra, Comaster, and Phanogenia) ; and the group seems 

 to be cosmopolitan in its distribution. 



ORDER CYSTOIDEA. Body generally spheroidal, pedunculate 

 or sessile, enclosed by calcareous articulated plates, some of which 

 are usually porous and are connected with respiration, and perhaps 

 with reproduction also. Arms rudimentary, mostly reduced to the 

 pinnulce only. Reproductive organs contained within the interior 

 of the calyx. 



The members of this order are all extinct, and are entirely 

 confined to the Palaeozoic period. The body (fig. 107) was, 



Fig. 107. Hemicosmites pyriformis, one of the Cystideans. The right-hand figure 

 shows the upper surface of the calyx. 



typically, more or less spherical, and was protected by an ex- 

 ternal skeleton, composed of numerous polygonal calcareous 

 plates accurately fitted together, and enclosing all the viscera 

 of the animal. The body was in most cases permanently 

 attached to the sea-bottom by means of a jointed calcareous 

 " column," or pedicle, but this was much shorter than in the 

 majority of Crmoids, and was rarely altogether absent. Upon 

 the upper surface of the body were two, sometimes three, 

 apertures, the functions of which have been a matter of con- 

 siderable controversy. One of these is lateral in position, is 

 defended by a series of small valvular plates, and is believed 

 by some to be the mouth, whilst by others it is asserted to 

 have been an ovarian aperture. The most probable view, 

 however, is that this valvular opening is really the anus. The 

 second opening is central in position, and it is believed by Mr 

 Billings to be the "ambulacral orifice," as it is always in the 

 centre of the arms when these are present. The third aper- 

 ture is only occasionally present, and its true functions are 

 doubtful. 



