124 



THE CRINOIDEA 



The enlargement of anal structures was not the only factor in 

 the modification of the typical pentamerism. Allusion need not 

 be made to the (apparently sudden) dropping of a radius to 

 form Tetracrimis (p. 153), or the duplication of the radii to form 

 Promachocriniis (p. 195), and similar cases. Nor need more be 

 said as to the enlargement of certain radii (e.g. Pisocrinus), the 

 bisection of others (e.g. Heterocrinus), and so forth, since in these 

 cases the outwardly symmetrical appearance of the cup usually 

 remained unaltered. But certain factors, probably of physical 

 environment, such as currents and direction of food-supply, or 

 possibly connected with locomotion, have at different times pro- 

 duced similar results in different families. A bending over of the 

 cup, accompanied by diminution of certain radials, was common 

 in Eugeniacrinidae, as well shown by Jaekel (1891). In the 

 remarkable Calceocrinidae the crown was bent towards the right 

 posterior interradius, and far-reaching changes brought about in 

 both cup and arms (p. 148). Even in an unattached species, 

 apparently of Agassizocrinus, similar growth of one side took place 



at the expense of the other. These 

 cases are comparable to the irregu- 

 lar Eublastoids. 



Concomitant with modifications 

 in the dorsal cup were modifica- 

 tions in the Tegmen. Just as 

 brachials entered the cup, so their 

 covering-plates (Amb) entered the 

 tegmen, prolonging the food- 

 grooves over its surface. And cor- 

 responding to interbrachials in the 

 cup, there arose interambulacrals 

 (iAmb) in the tegmen (Fig. 

 XXXII.). 



Other changes that took place 



Tegmen of Marsipocrinm radiatus, show- are difficult to describe without 

 ing incorporation of ^1/16, ambulacrals, with r : t v, p nnpqHnn nf thp hrvmo 

 iAmb, interambulacrals, between them; raising tne qUC 



Q f fae plates covering the 

 r o 



Amb 



JAmb 



FIG. XXX 11. 



an marks anal interradius, with its larger 

 interambulacrals. (After Lil.jevall in 



Wachsmuth & Springer. 1897.) x I- mouth. In Antedon five inter- 



radial plates (0) are developed 



before the radials and at the same time as the basals, upon which 

 they rest (Fig. XXXIII. 1). Between these two circlets appear 

 the radials, upon the shoulders of which the five adoral interradials 

 then rest (Fig. XXXIII. 2), forming a pyramid closed over the 

 oral centre, but soon opening at the apex to expose the entrance 

 to the mouth (tentacular vestibule). The posterior of these plates 

 surrounds the hydropore. At a more advanced stage they become 

 separated from the radials by ambulacrals and interambulacrals 



