UINTACEINUS: ITS STEUCTURE AND RELATIONS. 49 



There remain for consideration the two great genera of the free 

 floating ComatulaB: Antedon and Actinometra. Antedon has a central or 

 subcentral mouth, with plated ambulacra, and covering plates over the 

 food grooves usually extending throughout the arms and pinnules. Good 

 illustrations of this are found in several figures on Plate LV. of the 

 Challenger Report on the Stalked Crinoids, and the Report on the Coma- 

 tulae, Plates IX., XIL, XXXVIII., XL., XLI. It is essentially the disk of 

 the Pentacrinidse. 



Actinometra belongs to a type different from any other hitherto found, 

 either recent or fossil, in the structure of its disk. Instead of having the 

 mouth at or near the centre, as in all other known Crinoids, stalked or 

 unstalked (with the possible exception of the highly specialized Fistulata, 

 the complete structure of whose tegmen is not known), with the ambulacra 

 radiating about equally to the arm bases, it is situated excentrically, either 

 close to the margin of the disk, or at some point nearer to the centre, and 

 the centre of the disk is occupied by the anal tube. There are a variable 

 number of unequal ambulacra, at least two of which pass around on either 

 side in a broad curve enclosing the anal tube. These two groove-trunks 

 are often very long, and frequently each traverses nearly the semi-circum- 

 ference of the disk. Two or more other short groove-trunks run to the 

 lateral and anterior rays. Sometimes one or more arms are unpro- 

 vided with grooves. The ambulacra are open, shallow grooves in the 

 perisome, without plating or skeleton of any kind. P. H. Carpenter, 

 in his well-known paper '^ On the genus Actinometra,'' ^ says that not 

 unfrequently the posterior divisions of the two anterior ambulacra " unite 

 for a longer or shorter distance with the two large aboral groove- 

 trunks, to form an open horseshoe-shaped curve bounding the anal 

 area. The position of the anal tube in this area, and also with regard 

 to the whole surface of the disk, varies somewhat with the position of 

 the mouth; it is rarely, if ever, absolutely central. Its appearance differs 

 very much, according as it is full or empty; sometimes its aperture 

 is so completely closed as to be scarcely discernible, though the tube 

 below is widely distended ; and sometimes the aperture is patent, with 

 its edges erected and crenate, and the tube leading to it quite shrunk 

 and flaccid.'' 



A characteristic example of these structures is seen in Actinometra strata, 



* Trans. Linn. Soc, 2nd. Ser., Vol. II., p. 30. 

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