MONOGRAPH OF THE EXISTING CBINOIDS. 343 



a very much more perfect pentamerous symmetry, never possessing an anal or a 

 i-adianal. 



In the young of comatulids before the formation of the centrodorsal we find 

 what is essentially a highly dovelopod palirozoic type; the column is composed of 

 an indefinite number of similar colunmals, and the anal area is dill'erentiatcd from 

 the other interradial areas by the occurrence of a large radianal ; furthermore, the 

 plates of the calyx are large and entirely enclose the visceral mass, while the arms 

 are very short. 



The secondary bilateral symmetry of the Comasteridse has nothing whatever 

 to do with the bilateral svmmetrjr of palfEozoic forms, but results from the enormous 

 development of the digestive tube, which has shoved the mouth first to a marginal 

 position and then to the right, so that it comes to lie between the bases of the anterior 

 and of the right anterior post-radial series (figs. 21, 25-28, p. 69; see p. 152). This 

 appears to have been very suddenly acquired, as it is by no means univereal in the 

 family. 



The course taken by the mouth across the disk m the developing young of 

 species of this family shows that this character has been acquired very recently. 

 Until a considerable size is reached the mouth is central, just as in the correspond- 

 ing yoimg of Antedon. After the disappearance of the orals the mouth moves from 

 this central position to a position at the base of the anterior post-radial series, 

 and then laterally toward the right until it comes to rest on the margin of the disk 

 midway between the bases of the anterior and of the right anterior post-radial series. 

 Originally the species of the ComasteridiB possessed a disk resembling that erf 

 Antedon, as many of the species still do, and as all of the others do until a consid- 

 erable size is reached. 



The many-coiled type of digestive tube occurs only in such species of Coma- 

 SteridaB as are confined to shallow water and to more or less muddy bottoms; species 

 of the deeper and clearer water all possess the usual so-called endocyclic type of 

 disk. We thus naturally infer that the ingestion by the shallow-water forms and 

 by those inhabiting muddy bottoms of a large amount of inorganic material and 

 the use of a very large percentage of plants with highly developed skeletons as 

 food has caused, or perpetuated, a sudden development of the intestine. 



In the pelagic crinoids, such as Marsujntes (fig. 565, pi. 7) and Uintdcrinus, 

 the calyx is able to maintain a close approxunation to its primitive form, modified 

 only by an induced strengthening and bracing of the unions between the com- 

 ponent ossicles m types in which the arms are very long and heavy, thereby sub- 

 jecting the calyx to a considerable strain. 



The arms of Marsupites are, so far as we are able to judge, short and light, so 

 that in this genus a calyx showing a close approach to the most primitive possible 

 form of the pentamerous typo, upon which the later fossil and the recent crinoids 

 are constructed, is found. In Uintacrinus, on the contrary, the arms are excessively 

 long and heavy, and the strain which these long and honry arms exert upon the 

 cal}-x is counterbalanced by a reduction in size of the calyx plates anil by the 

 incorporation in the body wall of numerous brachials and pinnulars. so that the 

 mechanical stress is taken up by a network of small sutures running in ever\- direc- 



