96 LIFE BY THE SEASHORE. 



elytra, or scales, are present, and these elytra are un- 

 doubtedly nothing but metamorphosed dorsal cirri. We 

 thus see that though Polynoe has so little apparent resem- 

 blance to a " worm," yet it is in all essentials of structure 

 similar to Nereis. It is a great part of the interest of 

 worms that they show in this way how structures may be 

 modified and metamorphosed to fulfil different functions, 

 and satisfy changed needs. The elytra are hardened, horny 

 structures, and must serve to protect the organism, while 

 they are said also to be sensitive like the unmodified cirri. 

 We shall not consider the structure of Polynoe in further 

 detail, but may just notice that like Nereis it has a pro- 

 trusible introvert, in this case furnished with two pairs of 

 horny jaws. 



Related to Polynoe is the sea-mouse, a much larger and 

 handsomer form, which does not occur on the shore rocks, 

 but is often thrown up after severe storms. In shape it is 

 even less worm-like than Polynoe, for it has an oval- 

 depressed body with little sign of segmentation visible 

 externally. Either because of its beauty, or because it 

 may be practically supposed to be born of the foaming 

 breakers which toss it on the beach, it is named after 

 the fair goddess who* rose from the waves, and is called 

 Aphrodite aculeata. The body is densely covered with 

 bristles and hairs, which form a dense felt over the 

 scales, and at the sides of the body gleam with brilliant 

 iridescence, changing with every changing ray of light. The 

 sea-mouse may reach a length of six inches, but is usually 

 considerably smaller. The peristomium is remarkable 

 because it has shifted in front of the mouth, and bears 

 two typical parapodia a very unusual condition. The 

 scales number fifteen pairs, as in Polynoe imbricata, and 

 are similarly arranged, but they are not visible until the 

 dorsal mass of hairs is removed. The sea-mouse is a very 

 interesting worm, for it combines wonderful beauty of 

 colouring with ugliness of form. It is generally found 

 among weed and rubbish on the shore, and as one turns 

 over the debris its brilliant hues suddenly flash out in all 

 the colours of the rainbow. Partly, as I think, because of 

 the unexpectedness of the colouring, partly because many 

 people have little appreciation of form in the lower animals, 



