MONOGRAPH OF THE EXISTING CRINOIDS. 313 



plates in the lateral interradial perisome. In Mastigometra and Antedon (figs. 

 1018, pi. 7, and 1145, 1146, pi. 23) they occur as a small rounded group of from half 

 a dozen to a dozen plates in the interradial angles between the IBr,. 



ORGANIC BASE OP THE SKELETON. 



In general, the organic base of the skeleton forms a network of more or less 

 narrow meshes, between which the calcareous portions of the skeleton lie. Some- 

 times the spaces in this network are quite uniform, as in the calyx of Isocrinus 

 decorus, but often wider and narrower, elliptical, and circular meshes are found 

 indiscriminately in close proximity, as in the arms of Antedon. They are always 

 sharply outlined. 



The network is composed of the processes of star-shaped cells with at least 

 two branches which lie most commonly at the nodes, though present in other places. 



In the organic base of the skeleton two types of cells can be differentiated. 

 The cells of the first type are spherical or somewhat elongate with dark nuclei 

 surrounded by a small quantity of plasma. These cells give off processes in the 

 shape of fine fibrillse which branch extensively, often forming an irregular entangle- 

 ment with the fibrillfe of the neighboring cells. 



The cells of the second type probably have to do with the deposition of the 

 calcareous material. In the resting stage in the older skeletal structures they are 

 of very characteristic form, ovoid or almost spherical, with a usually elliptical 

 nucleus which lies close to the cell border, mostly at the more pointed pole. They 

 occur everywhere where calcified connective tissue is present, or is in process of 

 formation. 



In the regenerating arm where skeletal elements occur there are found 

 spherical mesenchyme cells which divide actively and possess a large nucleus with 

 a prominent nucleolus. As these cells increase in size the nucleus becomes some- 

 what smaller and stains so deeply that no details are visible in it. At the same 

 time there appear in the cell plasma granules of the same color as the nucleus 

 which, apparently consisting of chromatin, collect in larger or smaller masses. 

 Immediately a small vesicle may form, which increases in size and occupies a 

 large portion of the cell, though usually there appear in these cells a great number 

 of vesicles of different sizes, but always spherical. In older developmental stages 

 the nucleus is only rarely to be found and is of more or less indefinite form. 



Like the preceding, these cells appear to be concerned with the construction 

 of the calcareous elements. It is noticeable that the larger vesicles within them 

 show much similarity to the vesicles in the chambered organ. 



The calcareous substance in the ossicles of the arms and pinnules often projects 

 in the form of needle-like points through the epidermis. In Heliometra glaeialis 

 these extruding points may be as long as 0.02 mm. 



CONNECTIVE TISSUE. 



In the crinoids most of the connective tissue is found within and about the 

 skeletal elements. This connective tissue has already been described. 



