COMPARATIVE ANATOMY 



CHAP. 



368 



the five interradii there are several (twelve) interradials, the arrangement of which 

 is shown in the figure. The anal interradius is hardly distinguishable from the 

 other interradii. The distichals or secondary brachials are connected by smaller 



The perisomatic skeleton of the dorsal cup of Actinocrinus (Fig. 291, p. 329) 

 is very like that of Gilbertsocrinus ; but the anal interradius is much larger than 

 the others, and its plates are divided into two lateral groups by the intercalation of 

 a vertical 'row of anal plates. This is also the case in Batocrinus (Actinocrinida) . 

 Here, however, not only the 5x2 primary brachials and the 10x2 secondary 

 brachials, but also the 20 x 2 tertiary branchials are incorporated into the dorsal 

 cup. In Strotocrinus (regalis) an extreme form is found (Fig. 320). The calyx 



& 



Fro. 320. Strotocrinus regalis (after Wachsmutli and Springer). The apical border. The 

 conical portion of the dorsal cup is broken away (as far as the distichals di) and shows the 

 t.'-iiH-n with the anus, the mouth and the food' grooves. The dotted lines denote the manner of 

 branching of the fixed arms, an, anus ; If, fixed joints of the arms, which form the border ; B/, the 

 free arms which run out from the edge of the border ; ire, interambulacral region of the tegmen 

 calycis ; am, ambulacra ; pf, fixed pinnule. 



is very large. The dorsal cup consists of a small conical portion above the stalk, 

 followed by a border spread out horizontally. In each radius each radial is followed 

 by two primary brachials. The second costal is in each case followed by the two 

 secondary brachials (making ten in all). Up to this point the above mentioned 

 plates, together with the apical system, form the conical part of the dorsal cup. 

 The plates which follow form the horizontally expanded border. Each distichal is 

 followed by a principal row of (six) plates, which runs radially to the edge of the 

 border, where the last plate carries a free arm. Accessory rows branch alternately 

 from these principal rows, three on each side. These also run to the edge of the 

 border, and the last plate of each row carries an arm-branch. Seventy free arm- 

 branches in all thus rise from the edge of the border. In the interradii, in the 

 intcidistichal regions, and between all the further branches of the fixed arms, inter- 



