MECHANICAL PROTECTION AND POISONS 377 



primarily in the cytoplasm, but in a vacuole space which is bounded by 

 another distinct membrane, the vacuole membrane. The walls of this 

 membrane are continued from its sides down to the lower part of the cell 

 by a separate wall of membrane. Between the capsule and the vacuole 

 membrane lies a very small amount of fluid in the mature cnidoblast. 



One other important organ is developed in the cytoplasm in connec- 

 tion with the nematocyst. This is a pointed, chitinous rod called the 

 cnidocil. This rod projects distally beyond the surface of the surround- 

 ing epithelium, and, when touched, it acts so as to stimulate the cell, con- 

 tract the capsule, and force the thread to evert. This eversion is a very 

 efficient method of causing the thread to penetrate the body surface of 

 an enemy. Spines are placed on the inside of the folded or invaginated 

 thread so that when it is everted they will come out, point forward, and 

 penetrate a hard body first, thus making a way for the softer thread to 

 follow. Thus, the method of eversion does not involve any traction 

 or friction, which the thread is too delicate to bear. Once in the victim's 

 body, the spines are thrown backward and serve to retain the thread. 

 In passing into the victim the thread carries a poison with it. 



A very different type of integumental organ of offense is found in 

 some of the Echmoderms, the sea-urchins or Echinoidea. In these ani- 

 mals the body is covered with spines which project from the surface and 

 are movable on a knob-like process of one of the plates with which this 

 animal's skin is provided. In some forms the spines are comparatively 

 harmless, but the spines of a very common, black urchin, Diadema, 

 found on all tropical coasts, are of tremendous length and exceedingly 

 sharp. 



Such a spine consists of a core of mesodermal origin and an outer 

 integumental layer. This outer layer originally has an epithelium con- 

 tinuous with the rest of the body, but when the spine is mature this epithe- 

 lium is rubbed off, leaving the hard, cortical layer of lime for an outside 

 covering. Near the base, the epithelium persists and, in Diadema, is 

 subject to interesting modifications (see Chapter XIII, on Eyes). 



The internal part of a spine of Diadema (Fig. 344) consists of a long, 

 central canal for circulation surrounded by a mesodermal tissue covered 

 with epidermis. The tissue about the canal is built out into^a number 

 of radial plates which are attached longitudinally to the canal-bearing 

 core. These plates meet folds of the integument which project inward 

 and thus form a series of tubular spaces in which the lime tissue of the 

 spine is laid down. At first this lime is an almost solid rod, but as the 

 spine grows, a connective-tissue reticulum is formed and acts as a basis 

 for the additions of lime as shown in Figure 344 at cal. 



The base of the spine is flexible, owing to a disk-shaped "region in 

 which no lime is deposited, and where the connective tissue is developed 



