SECT. v. ERR ANT I A. 1 59 



edges like the scales of a fish. They are well developed 

 in the Aphrodita hystrix, or the Sea Mouse of fisher- 

 men, and its congeners. That Annelid, which is an 

 inhabitant of European coasts, is thicker and broader 

 than other sea- worms. The two rows of overlapping 

 shields on its back, and the quantity of iridescent 

 hairs, cirri, and other appendages covering the body, 

 is so great as to form a kind of felt or fur like the 

 skin of a mouse. The members of this genus of sea- 

 worms have no gills properly so called; the only ex- 

 ternal sign of respiration is a periodical elevation and 

 depression of the shields on their backs by the action 

 of a complex system of muscles. The thick covering of 

 felt on the body of the worm below the shields becomes 

 filled with water during their elevation, which is ejected 

 forcibly at the posterior end of the body during their de- 

 pression. Although the water does not penetrate the 

 thin skin on the back of the worm, its oxygen does, and 

 is accumulated in the colourless liquid in which the 

 stomach floats ; and from it the blood, which is of a pale 

 yellow colour, receives its oxygen. The feet of the worm 

 are fan-shaped groups of sharp glassy bristles enclosed 

 between two plates, which prevent them from hurting the 

 animal when it puts them out or draws them in. The 

 Aphrodita is male and female : the eggs escape through 

 pores in the female, and are received in a kind of pouch 

 beneath the dorsal shields till hatched. The embryo 

 is an oval locomotive mass, with groups of cilia, and 

 indications of an eye-speck: after swimming about for 

 twenty-four hours, the segments begin to be developed. 



Worms of the genus Polynoe have also two rows of 

 shields on their backs, but they are studded with trans- 

 parent oval bodies on short stems, supposed to be 

 organs of touch. The filiform tentacles and antennae 

 that are developed between the shields, as well as the 

 cirri or curly bristles of the feet, are likewise covered 



