MECHANICAL PROTECTION AND POISONS 



375 



DAVIES, H. R. " Die Entwicklung der Feder und ihre Beziehung zu anderen Integumen- 



tatgebilden," Morph. Jahrbuch., Band XV. 

 NUSBAUM, JOSEF. "Zur Histogenese der Lederhaut und der Cycloid Schuppen der 



Knochenfische," Anat. Anz., Band XXX, Nos. 1112, 1907. 

 VITZON, ALEX-NICH. " Recherches sur la struct, et format, des integuments chez les Crus- 



tecis Decapodes," Arch, de Zool. Exp. et Gen., t. x., pp. 451-576, pis. XXIII-XXVIII, 



Paris, 1882. 

 HERRICK, F. H. "The American Lobster," U.S.F. Comm. Bull, for 1895, pp. 1-252, 



plates A-J and 1-54. 



OFFENSIVE MECHANICAL PROTECTION AND POISONOUS FLUIDS 



Offensive mechanical protection is a function of many kinds of 

 epithelia and the products of these epithelia as well as dermal structures 

 associated with them. It is but remotely 

 removed, so far as the structure of the tis- 

 sues that perform it is concerned, from the 

 passive, mechanical protection treated of 

 in the last section. A series of merely 

 ornamental points on an insect larva 

 might be developed by selection or other- 

 wise into stinging organs and spines. Also 

 the same sort of development of the har- 

 dened outer layer of the stratified epithe- 

 lium that results in a downy hair or 

 feather is sometimes used to produce 

 claws and horns. The principal factor 

 that leads the writers to separate these 

 mechanically offensive organs from the 

 simple protective structures, histologically, 

 is the fact that many of them are associ- 

 ated with poison glands. The associated 

 poison glands will also be described at the 

 same time. 



We shall first consider three intracel- 



FIG. 342. Part of a vertical section 

 of a triclad worm. The outer layer 

 consists of a simple layer of colum- 

 nar, epithelial cells containing tri- 

 chocysts in their distal cytoplasm 

 and showing a modified and pro- 

 tective border. Beneath this the 

 underlying tissues have become ar- 

 ranged in layers with reference to 

 the body surface. (After PARKER 

 and HAS WELL.) 



lular forms; the trichocysts of Infusoria (see Fig. 245); the rhabdiles of 

 turbellarian worms, and the nettle-cells of ccelenterates. Secondly, the 

 extracellular forms from columnar epithelia and the multicellular forms 

 found developed from stratified epithelia. 



The simplest forms of such organs are the rod-like trichocysts of 

 Paramcecium and other Infusoria, and the so-called rhabdites and stylets 

 found in the turbellarian and nemertean worms (Fig. 342). These are 

 short rods pointed at the outer end and formed in the cytoplasm, usually 

 of the cells that contain and use them. They may be formed by internal 



