A MARVELLOUS STRUCTURE. 153 



As it crawls, tlie li^lirodite usually elevates the 

 tail, which is so folded together as to form a deep 

 groove beneath. By watching this, we see now and 

 then ejected a stream of water with considerable force. 

 I found that the jet occuiTed once in twenty-five 

 seconds, with punctual regularity. This is a respi- 

 ratory act ; the grooved orifice through which the jet 

 is poured is not the termination of the intestine, as 

 we may at first suppose, but the exit of a capacious 

 chamber wdiich is external to the body, though con- 

 cealed. 



A very marvellous and quite unparalleled structure 

 here comes into view. If we take a Sea-mouse into 

 our hand, we see the whole breadth of the back occupied 

 by a woolly substance, closely resembling felt, and 

 formed by the interlacing of fine hairs. If we insert 

 a penknife into the tail-gi'oove, and slit up this felt- 

 like cover, we expose an ample cavity running the 

 whole length of the animal, the floor of which is the 

 true skin of the back, on which are set two rows of 

 large overlapping plates, or membranous scales 

 {elytra). 



The dense tissue of interwoven hair resembling felt 

 acts as a filter for the water to be respired, straining 

 ofi" the earthy particles held in it, which thus accumu- 

 late in its substance, and impart that peculiar dirty 

 appearance which it possesses. The scales, according 

 to Dr. Williams,* are periodically elevated and de- 

 pressed. In the former action, the water permeates 

 the felt, and fills the vacuum formed between them 

 and the back. As soon as it is full, they collapse ; 



* Eeport on the British Annelida, 200. 



