378 



HISTOLOGY 



centrally into a ligament. The peripheral mesodermic tissue of this 

 same region forms a circular sheet of longitudinal muscle fibers which, 

 by pulling on one side or on the other, is able to move the spine in any 

 desired direction. Around the base of most echinoderm spines is a 

 ring of dermal tissue in which nerve cells and their fibers cause a con- 

 siderable thickening. This is cut tangentially in the figure. 



mes. pi. 



FIG. 344. Section through the base of a poison spine of the echinoderm, Diadema setosum. 

 mes.pl., mesodermal plates radiating from the central canal (here occupied by one of the 

 amoeboid blood cells); x, spaces occupied by the youngest plates of lime; cat., organic base 

 of the older calcareous tissue; int., outer integument; mus.f., some of the muscle fibers 

 which move the spine; n.in., integument containing the mass of nerve tissue. X 520. 



The spine makes a painful wound, and its outer tissues probably 

 secrete an irritating fluid. In another form, Msthenosoma, the tip of the 

 spine is enormously enlarged and its upper point invaginated into a 

 pocket. From the base of this pocket arises a spicule which is a continua- 

 tion of the core of the spine and the whole sac is filled with a poisonous 

 fluid secreted by the lining cells (Fig. 345). When the delicate head of 

 the spine Is touched by any creature, the top breaks or caves in, and the 

 spicule is forced into the victim's flesh. At the same time the muscles 



