IDOTEA TRICUSPIDATA. 



381 



on each side of the body, whilst other specimens are 

 marked, often irregularly, with large pale yellow or orange- 

 coloured patches on the body and tail. According to our 

 own experience, the colour of the animal is dependent 

 upon that of the weed on which it lives. Those that 

 live on the black fucus are generally very dark purple, 

 while those that we find on the green Algae are brightly 

 verdant ; and it has always been our opinion that this 

 change was due to the food, as we have little doubt but 

 that they are vegetarians in diet, and that the depreda- 

 tions on fish is that of their marine companions. 



The accompanying woodcut exhibits different varieties, 

 showing the comparative paucity of the articulations in 

 the flagellum of the lower antennas in young individuals, 

 the length of the three individuals whose antennae are 

 delineated being indicated by the three adjoining lines, 

 whilst the other figures exhibit variations in the form of 

 the terminal segment of the body, with the length and 

 size of the respective specimens in the British Museum, 

 the first, third, and fourth of which are from Cornwall 

 (B. M. Coll. No. 50, 94, and 45, 139), and the large 

 second one from Cardiff (No. 56, 73). Some very fine 

 specimens taken off Cornwall have the caudal extremity 

 terminating in three well-defined cusps, as also one sent 

 to us by Mr. Gosse from Torquay. 



Specimens having the extremity of the tail truncate, 



