200 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The ancestral criiioid was developmentally and phylogenetically parallel to 

 such of the later ochiiioids as possess a well-developed central plate, as well as com- 

 pound anibiilacrals and well developed auricles. 



The inversion of the crinoid as compared with the echinoid brought the central 

 plate into contact with the sea floor and, the central plate being a secondary peri- 

 somic structure, and therefore an inert calcareous element of great potential varia- 

 bility the shape and thickness of which are in no way confined within narrow 

 limits by ])hysiological, mechanical or phylogenetic limitations, it became attached 

 to the sea floor by a simple increase in thickness. 



The facility with which organisms with calcareous skeletons belonging to 

 normally free groups become attached is well illustrated in many diverse moUuscan 

 families, among the barnacles, the foraminifera, and numerous other classes of 

 animals, all of which furnish cases strictly parallel to what we find in the crinoida 

 among the echinoderms. 



Attached by the central plate, our theoretical ancestal crinoid has two possible 

 courses to follow: (1) It may increase the area of its attached base, or (2) it may 

 increase its thickness, thus forming a column. 



Among the recent forms the first possibility is realized through reversion in the 

 young of IIolopus as figured by Mr. jVlexander Agassiz (fig. 514, pi. 1) ; the base has 

 spread out enormously so that the animal presents a striking similarity to certain 

 low species of sessile barnacles, the ten arms being countersunk, as it were, in a 

 depression at the apex of a broad low truncated cone. The second possibility is 

 exemplified among recent forms by the adult Holopus (fig. 517, pi. 1); the base, 

 instead of further spreading out, gradually becomes thickened, so that the animal 

 is raised up for a considerable distance on a thick stalk. 



The attachment of IIolopus, incidentally, is singularly suggestive in reference 

 to the question of the phylogeny of the crinoids, and therefore of the echinoileims in 

 general. All the evidence — anatomical, structural, and embrj^ological — points to 

 their having derived from a generalized phyllopod crustacean ancestor tlirough the 

 barnacles, just beyond which they find their logical position. In the young IIolopus 

 we find du])licated the attachment characteristic of the sessile barnacles, while in 

 the adult we find the typical attachment of the stalked barnacles. 



Now a rigid calcareous stalk like that of IIolopus is lunited m its availability 

 for elongation; if it should grow to more than three or four times as long as the 

 minunum diameter, it would rapidly become exceedingly brittle and liable to fracture 

 by the contact of the animal with other organisms, or even from the effect of wave 

 motion. 



There are, again, two possible lines of development: (1) Th(^ animal ma}' break 

 off and thus secondarily become free, or (2) the column may break in so far as the 

 calcareous substance is concerned, yet remam in continuity through the organic 

 base, thus developing an articulation which would admit of a verj' consider al)le 

 additional elongation — at least double that permitted by the original column. 



Such a fracture of the column must not be regarded as an actual phj-sical 

 fracture, but as a morphological fracture induced during the development of the 



