132 A NEW ZEALAND NATURALIST'S CALENDAR. 



because, though coloured in conformity with their sur- 

 roundings, it is not protection they need in the water but 

 such similarity of hue as will enable them to get near their 

 prey without attracting notice ; but were they to move out 

 from the shelter of some friendly stone into the open they 

 would run the risk of being picked up by some passing 

 gull. Against such an enemy their strong nipping claws, 

 which are such excellent offensive and defensive weapons, 

 would prove of no avail. 



Or perhaps we may disturb a colony of porcelain crabs* 

 small bluish fellows with disproportionately big, angled, 

 and flattened claws. If we take one up between the finger 

 and thumb, gripping it by the back of the carapace, and 

 offer another finger to its claws, it will astonish vis, small 

 as it is. Most singular of all are rather large crabs t with 

 feeble claws, covered on the back with tufts of hair, serpulae, 

 and seaweeds, sometimes carrying a bunch of sertularians, 

 a barnacle, or even a sea - anemone, to increase their 

 resemblance to a seaweed-covered stone. This is a defen- 

 sive, not an offensive, device. They hope to escape notice 

 by their resemblance to the ground on which they move 

 or rest. But all Nature abounds with such devices, and we 

 meet with adaptations of the kind in every realm. When 

 these animals moult or change their shells, which they do 

 periodically, the new shell is smooth, but with their claws, 

 they seem to plant other organisms on their backs, so as to 

 secure this covering growth as quickly as possible. 



Many of the plants and most of the animals of the rock- 

 pools can be kept in an aquarium, and their beautiful forms 

 and movements studied leisurely. But to keep an aquarium 

 at all takes some practice and due consideration of the 

 natural conditions under which the denizens of the pools 

 live. The water must be kept at uniform density by daily 

 additions of a little fresh water, it must have constant 

 aeration, a judicious mixture of plant and animal life, and, 

 most important of all, too many living things must not be 

 crowded into a small space. By attention to these details 



* Petrolisthes elongatus. f Paramithrax latreilli. 



