Crustacea 207 



gone through by all Crustacea, has been observed to take 

 place in these forms with surprising frequency. In the last 

 mentioned species, Simocephalus sima, the moult takes place 

 with considerable regularity, every four or five days in the 

 case of the adult, and probably still more often in young and 

 growing specimens. 



The relations of these Cladocera to the surface-film of 

 water are worth attention. In cases where the creatures 

 have been kept captive, it is no infrequent thing to find 

 that certain specimens have become entangled in the surface- 

 film and are unable to sink below the surface. Whether 

 such an occurrence is ever to be met with in nature, and 

 if not, why it is not the case, are points which remain to 

 be investigated. In this connection, however, it is interesting 

 to note that one of the Daphnidae Scapholeberis deliber- 

 ately makes use of the surface-film for support, attaching itself 

 by the modified hairs on the ventral margin of its shell. 



Parthenogenesis, or reproduction without fertilisation, 

 during certain periods of the year, is a characteristic feature 

 of Cladocera, and these common forms are among the most 

 prolific of them all. Daphnia pulex may have frequently as 

 many as 40 eggs at a time in the brood chamber, and only 

 four or five days are necessary for the complete development. 

 As of course each offspring soon becomes capable of continuing 

 the process, it will be seen that the rate of increase must be 

 most prodigious. A calculation, based on numbers below the 

 average, gives the startling result that the descendants of 

 a single female, after 100 days, would number one and a half 

 billion. After fertilisation, which takes place in the autumn, 

 and occasionally at other periods of the year, the female 

 produces a small number of eggs which are enclosed in a 

 protective case, the so-called ephippium. 



A few other forms of Cladocera are perhaps worthy of 

 notice. Chydorus sphaericus is one of the commonest of all, 

 occurring in most of the ponds and ditches, though from its 

 small size it is easily overlooked at first sight. It is a tiny 



