150 



ACTINOZOA. 



other small foreign body, introduced into the 

 vesicle, is quickly and tenaciously secured. I n 

 their natural situations these creatures are often 

 completely covered by fragments of shell, gravel 

 or sand, attached to their bodies by a peculiar 

 viscid secretion, in the production of which these 

 warts are, perhaps, concerned. 



Or, the epidermic secretion may give rise to a 

 distinct membranoid coat, protecting the integu- 

 ment of the animal, from which it is at times cast 

 off by what may be termed a process of sloughing. 

 Such a membrane in Cerianthus Mr. Cfosse states 

 to be " wholly composed " of altered cnidse, which 

 intertwine one with another to form a wide tube, 

 investing the entire surface of the column. Here 

 the connection of the tube is so loose that it can at 

 any time be removed without much inconvenience 

 to the animal, but, in other genera, a more 

 adherent covering may be found. In Adamsia 

 the base excretes a delicate, somewhat chitinous 

 membrane, which, upon occasion, may continue 

 its growth beyond the attached outline of its 

 possessor, and even form an artificial extension of 

 the peculiar surface which this genus is wont to 

 choose for its abode. 



The thread-cells of the Zoantharia have been 

 studied with great care by Mr. Grosse, who dis- 

 tinguishes four principal kinds of these bodies 

 by the titles of ' chambered, 5 * spiral,' ' tangled ' 

 and^ 'globate cnidse.' The chambered cnidse 

 (which are the most common) are of a long oval 

 form, the ecthorseum, which varies greatly in 

 length, presenting in all cases, thecomplex armature 

 characteristic of these minute weapons ; a number 

 of delicate barbs, or 6 pterygia ', being attached to 





