8o 



Introduction to Animal Morphology. 



motory, and usually generative organs. The outer 

 surface is rarely ciliated in the adult, except in 

 Ctenophora. In the protoplasmic ectoderm exist 

 scattered or clustered nettle-cells (cnidae or trichocysts), 

 (Fig. 9, B C), which are minute oval capsules filled 

 with fluid, and consisting of a fine homogeneous outer 

 layer, lined by a delicate membrane, which at the 

 apex is involuted into an axial tube, ending in a long, 

 coiled, barbed thread. The slightest contact ruptures 

 the outer layer, the tube becomes everted, and from its 

 summit the thread suddenly uncoils and becomes 

 rigid. The everted tube is armed 

 with three or more barbs (Fig. 9, C). 

 A Minpler form consists of a simple 

 sac and filament with no tube; 

 sometimes a second filament exists 

 in an oblong saccule at the barbed 

 end of the first thread. These are 

 used in benumbing their prey by ad- 

 hesion, penetration, and possibly by 

 some virus ; but each cell can only 

 act once. In development, they first 



! . .. 1-1 <-. The samo with the 



appear as clear particles, which soon thre 



Fig. 9. 



assume a double contour (and sometimes exhibit 

 amoeboid motions) ; then the spiral thread appears 

 within, and the cell becomes superficial and ready to 

 act. They are most numerous on the tentacles, and 

 often regularly arranged. 



Sexual reproduction occurs in every species, and 

 the term individual is applied to the total product of a 

 single impregnated ovum : when this consists of parts. 

 which sustain separate existence, each part is called a 

 zooid. 



