Professor Herdman is of opinion that although the cerata may in all cases aid in respiration to a certain extent, yet that extent is so small as to be left out of consideration altogether. He regards the cerata in both the two classes mentioned above as “of primary importance in giving to the animals, by their varied shapes and colours, appearances which are in some cases protective, and in others conspicuous and warning.” Thus, for instance, _Tritonia plebeia_, which is fairly abundant at Puffin and Hilbre Is., appears always to be found creeping on the colonies of a particular polyp, _Alcyonium digitatum_, and nowhere else. The specimens in each colony of the polyp differ noticeably both in the matter of colour, and of size, and of varied degrees of expansion. The _Tritonia_ differs also, being marked in varied tints of yellow, brown, blue, gray, black, and opaque white, in such a way as to harmonise with the varied colours of the _Alcyonium_ upon which it lives. The cerata on the back of the _Tritonia_ contribute to this general resemblance. They are placed just at the right distance apart, and are just the right size and colour, to resemble the crown of tentacles on the half-expanded polyp. Similarly, _Doto coronata_, which, when examined by itself, is a very conspicuous animal, with showy, bright-coloured cerata, is found by Professor Herdman to haunt no other situations but the under side of stones and overhanging ledges of rock which are colonised by a hydroid, known as _Clava multicornis_. The _Doto_ is masked by the tentacles and clusters of sporosacs on the zoophyte, with whose colouring and size its own cerata singularly correspond. A similar and even more deceptive correspondence with environment was noticed in the case of the very conspicuous _Dendronotus arborescens_. In these cases, the colouring and general shape of the cerata are protective, _i.e._ they match their surroundings in such a way as to enable the animal, in all probability, to escape the observation of its enemies. According to Professor Herdman, however, the brilliant and showy coloration of the cerata of _Aeolis_ is not protective but ‘warning.’ _Aeolis_ does not hide itself away as if shunning observation, like _Doto_, _Tritonia_, and _Dendronotus_; on the contrary, it seems perfectly fearless and indifferent to being noticed. Its cerata are provided with sting-cells, like those of Coelenterata, at their tips, and its very conspicuousness is a warning to its enemies that they had better not try to attack it, just as the showy white tail of the skunk acts as a sort of danger-signal to its own particular foes. It is important for the _Aeolis_, not merely to _be_ an unpalatable nettle in animal shape, but also to be conspicuous enough to prevent its being experimented upon as an article of food, in mistake for something less nasty.