THE BAR CO DIN A 



77 



follows the outlines of the vesicles which form this zone of proto- 

 plasm, the resulting skeleton forms a reticulum. Growth may take 

 place more rapidly, however, at certain places, and spines, spicules, or 

 protuberances of one kind or another are the result. The usual form 

 of the network upori which the skeleton is deposited is an hexagonal 

 mesh, but this may become modified in numerous ways, the apertures 

 becoming either circular, polygonal, or elliptical (Fig. 41). 



When spines are formed, a secondary calymma may also be 

 developed, carrying with it the sarcodictyum, and the latter, in turn, 

 may give rise to a secondary skeleton outside of the first. This 

 process may be repeated until there are as many as six or seven acces- 

 sory skeletons. 



^ 



Fig. 41. Schematic figure illustrating the modifications of skeletons according to mechanical 



principles of deposition. [DREYER.] 



The secretion is supposed to collect in the interstices between alveoli as at (c), forming simple 

 spicules, or tri- and tetra-radiate spicules (3). Collecting in the lines of union of six alveoli, the 

 deposit takes the form of an hexagonal mesh (d), which, by the addition of more material, becomes 

 changed as at (a), (e), (f), and (g). 



A very interesting set of phenomena are connected with the 

 acanthin skeletons where the spicules are not deposited in the 

 calymma, but are formed at the centre of the central capsule, growing 

 out centrifugally into the extra-capsular plasm and resulting in a 

 skeleton of radiating spines. With a few exceptions these spines are 

 twenty in number, and are arranged in a certain geometrical order 

 which has been characterized as the Miillerian law. The points of 

 the spines fall in five circles parallel to the equator, and there are four 

 spines to each circle. The spines are named, according to this 

 scheme, polar, tropical, equatorial, sub-tropical, and sub-polar (Fig. 42). 



The form of the silicious skeleton is quite varied. In its least-dif- 

 ferentiated form, as in most Heliozoa, it is a mere collection of loosely 

 arranged spicules. In other forms a uniform covering of silica covers 

 the meshes of the sarcodictyum. Such a generalized condition of the 

 skeleton becomes modified in many ways, the main types being the 

 " sagittal ring," consisting of a simple ring of silica, like a girdle 



