CHAPTER XV 

 MOLLUSCS 



.s already mentioned, the molluscs of the sea are far more 

 numerous both in individuals and in species than those of fresh 

 water. The highest class indeed, the cuttles, occurs only in 

 the sea. The cuttles are by far the most interesting of the 

 molluscs, and it is unfortunate that they are generally difficult 

 to keep alive in the aquarium. They are all capable of swimming 

 rapidly, and are highly differentiated forms, whose needs are 

 difficult to satisfy in captivity. Many are indeed too large for 

 success to be hoped for, but such forms as the common squid 

 (Sepia), Loligo, or Ommastrephts, which may be found cast up 

 on the beach after spring storms, or are sometimes obtained 

 from fishermen, may be kept for a day or two in a tub of water 

 to demonstrate the essential points of structure. In some parts 

 of the coast the very pretty little octopus known as Eledone 

 cirrosa, which only reaches a few inches in length, occurs in the 

 rock pools, but it is not very easy to keep alive for long. The 

 points of interest about the cuttles are first of all the power of 

 colour change, which persists even after the animal is apparently 

 dead, so that a portion of the body of a cuttle which has been 

 torn to pieces by the gulls as it lay on the beach will still show 

 the phenomenon. To demonstrate this it is only necessary to 

 pass the hand over the skin, when the curious flushing and paling 

 can be observed. In a living cuttle under natural conditions 

 the colour shows a general resemblance to the surroundings, and 

 this doubtless protects the animal, or enables it to steal unper- 

 ceived on its prey, for the cuttles are predaceous animals. Another 

 interesting point, easily demonstrated in a captive Eledone, is 

 the power of throwing out a cloud of ink which darkens the 

 water. The method of swimming should also be observed. 

 Some cuttles have fins at the sides of the body and can swim 



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