494 



occur on the inner surface of the siphons (where the test is not covered 

 by the velum), on the margins of the apertures and to some extent on 

 the outer surface of the siphons and on the body generally. They seem 

 to be usually restricted to the siphons. 



Each spinule appears to be derived from a single test-cell of a kind 

 differing from the ordinary cells of the test. They are of somewhat 

 larger size and in some cases at least stain more deeply with eosin. They 

 pass to the free surface of the test and the exposed part of the cell wall 

 becomes converted into the spinule. They orient themselves with re- 

 Fig. XII. 



Fig. XI. 



-t.es- 



Cp~ 



Fig. XL Atrial walls of various Styelids. (X 13.) 1, Cnemidoearpa joannae ; 2,Pan- 



docia fibrosa; 3, Dendrodoa carnea, tes, test; vel, velum, ten, tentacle. 

 Fig. XII. Atrial walls of (1) Katotropa vancouverensis and (2) Paratona elata. (Xl3.) 



ference to the long axis of the siphon, the spine pointing toward the 

 outer opening of the siphon. 



These spinules do not occur in all Styelids, though doubtless the 

 peculiar test-cells are to be found in all. The spinules show a consid- 

 erable variety in form in the various species, but the general plan is the 

 same in all. The principal differences are due to the spinules of some 

 species developing farther than those of others. In the simplest form 

 there is a transverse thickening of the wall of the cell. This thickening 



