Searle, The Pond and its Inhahitnnts. [voT."^xxxt 



XXX TV. 



some of his captures — not always an easy thing to do, owing 

 to the absence of htcraturc on the subject. 



Water Fleas. 



One of the commonest inhabitants of our ponds is the 

 water flea. It received this name not because it was 

 related in any way to the lively insect which at times 

 proves so irritating to man, but because of the jerky manner 

 in which it progresses through the water. The water flea 

 belongs to that division of animals called Crustacea, of which 

 the shrimp and crayfish are familiar examples, and is placed 

 in that group of the Entomostraca called Cladocera. The 

 water fleas were among the first of the pond animals in Aus- 

 tralia to receive the attention of naturalists, several forms 

 being described and figured by the Rev. Robt. L. King as early 

 as 1851. One of these, which he named Daphnia carinata. 

 is of great interest to biologists on account of the variability 

 in the form of its carapace. Seven distinct varieties have 

 been recognized by specialists, one of which, var. lamellata, 

 possesses the additional distinction of being the largest known 

 Daphnia, its measurements being 8.3 mm. in length and 

 7 mm. across the cephalic part of the carapace. It is accordmgly 

 nearly twice as large as D. magna, which hitherto has iDeen 

 considered as the largest of all Daphnias. 



For the microscui)ist there is no ninrc beautiful object for 

 examination than Daphnia. It is so beautifully transparent 

 that we are able to examine through the carapace every 

 function of its existence. We see the rhythmic iDeat of its 

 leaf-like legs drawing fresh streams of water into its carapace 

 so that the blood is aerated and regenerated by the oxygen 

 dissolved therein ; and it might be said with truth that Daphnia 

 breathes through its feet. Its heart, placed in the middle of 

 its back, can be seen in rapid pulsation ; we can see the slit- 

 like valve open and close as the blood is drawn into and 

 forced out of that organ, and by observing the corjmscles 

 floating therein we may follow its course as in turn it bathes 

 the brain, stomach, respiratory organs in the legs, circulating 

 through the carapace, and flowing back to the pericardial cavity 

 to be again drawn into the heart, and once more sent circulating 

 through its body. 



Furthermore, if we have the patience of a true lover of 

 nature, we can watch with interest the formation of the eggs 

 in the ovary, and study the development of the young Dajihnia 

 from the segmentation of the egg to the expulsion of the 

 fully-developed young from the brood chamber of the mother. 



Daphnia, in common with the rest of the Cladocera, possesses 

 a single eye, of a compound nature, placed in the front of the 



