96 THE BIOLOGY OF THE SEA-SHORE 



the shores of Hawaii, near Hilo, and on those of Tobago 

 Island in the Gulf of Panama, was found a shrimp of the 

 genus Hymenocera^ not merely resembling red algae in colour, 

 but actually simulating their broad flat iridescent " leaves " 

 though, curiously enough, the animal was not living among 

 the algae. 



On British coasts a common example of protective 

 resemblance is afforded by the Porcelain crab (Porcellana 

 platycheles). Herdman (1892) notes that, on Puffin Island, 

 where it frequents the under surfaces of irregular fragments 

 of carboniferous Hmestone, the animal with its flat body 

 tends to take up its position in slight depressions on the 

 rock where the dirty greyish brown colour of its carapace 

 corresponds exactly with the muddy surface of the decaying 

 limestone. Moreover, to heighten the resemblance, the 

 tiny hairs with which the limbs and carapace are fringed 

 entangle mud and dirt, and so help to grade the body off 

 into the surrounding stone. 



Numerous cases of close resemblance to surroundings 

 have been recorded in British Nudibranch molluscs. Often 

 the resemblance is heightened by the form and coloration 

 of the " cerata." These are outgrowths of the body wall, 

 and a characteristic feature of Nudibranchs. According to 

 Herdman and Clubb (1892) they are of two distinct kinds : 

 they may contain diverticula of the liver, and be directly 

 concerned in digestion, e.g. Aeolis and Doto ; or they may be 

 simply lobes of the skin with no connection with the liver, 

 and no special functions to perform, e.g. Tritonia^ Ancula, 

 Dendronotus . The form Tritonia pleheia (fairly abundant at 

 Puffin and Hilbre Islands) is found creeping on colonies of 

 Alcyonium digitatum, and in no other situation. The polyps 

 of a colony differ markedly in colour, size, and degree of 

 expansion, and the Tritonia is found to differ accordingly. In 

 large measure the resemblance is due to the " cerata," which 

 are placed just the right distance apart, and are just the right 

 size and colour to resemble the crown and tentacles on the 

 half-expanded polyp. The brown Runcina hancocki is said 

 to crawl over brown mud and seaweeds, but to " avoid " 



