256 LIFE BY THE SEASHORE. 



Firth of Forth, especially in spring. Its colouring is similar 

 to that of Polycera quadrilineata, but there is only one 

 yellow line placed in the middle of the back. The yellow 

 tips to the processes are also often much less bright, the 

 animal at times being wholly white. It is easily dis- 

 tinguished from the preceding form by the arrangement 

 of the processes. These are absent on the head itself, but 

 the stalks of the dorsal tentacles each bear two. There are 

 three large bipinnate gills, and these are surrounded by a 

 circle of yellow-tipped processes, instead of the two of 

 Polycera quadrilineata. All these points are well shown 

 in the figure. The animals live well in confinement, where 

 they spend much of their time floating at the surface, back 

 downwards. 



Fia. 74. Ancula cristata. After Alder and Hancock. 



The next form, Dendronotus arborescens, is regarded by 

 many naturalists as the most beautiful of our sea-slugs. 

 Its name and its beauty are both due to the fact that the 

 back is furnished with numerous branched and brightly 

 coloured processes, which make the creature look more like 

 a dainty piece of seaweed than a living animal. Bright as 

 the colours are, they harmonise wonderfully with the reds 

 and browns of the corallines among which the animal lives, 

 so that it is by no means conspicuous in natural conditions. 

 Like most sea-slugs it is rarely if ever eaten by shore 

 animals, so that the colouring, although it resembles the 

 surroundings, can hardly be described as "protective," and 

 it is certainly remarkable that colouring of this kind should 

 be common among animals apparently rarely attacked by 

 others. 



