CRUSTACEANS 



if the first which constitutes the head be allowed to count as a single 

 segment notwithstanding its evidently composite character. The follow- 

 ing five segments are thoracic, and the terminal five form the pleon, a 

 part so called not with regard to the Copepoda but to other crustaceans 

 in which it is in fact what the word implies, the swimming factor of 

 the organism. In the Copepoda this generally ends in a pair of seti- 

 ferous processes known as the caudal fork or furca, but otherwise it 

 has no appendages, and the expression Gymnoplea means ' those that 

 have a naked pleon,' or in other words a pleon without appendages. 

 Contrasted with these are the Podoplea, or ' those that have feet on the 

 pleon.' But the awkward thing is that these do not really any more 

 than the others have such feet. What they do have is this. The last 

 segment of the thorax, instead of keeping with its own company, has in 

 a manner broken away, and tacked itself on to the group of pleon seg- 

 ments so that it gives the Podoplea not the reality but the look of 

 having a pair of feet (often very rudimentary ones) on the pleon. To 

 this division belong the family Cyclopidas, of which Cyclops fuscus 

 (Jurine) and C. bicuspidatus, Claus, were obtained respectively from 

 Burnham Beeches and a common adjacent thereto, and the family Ar- 

 pacticidae, of which Canthocampus pygmceus, Sars, was obtained from both 

 the last mentioned localities, and C. stapbylinus (Jurine) from Burnham 

 Beeches and Stoke Park. In the Diaptomidas one antenna, either the 

 right or the left, of the first pair is modified into an organ for clasping 

 the female, a geniculation being formed between the eighteenth and 

 nineteenth joints. In the Podoplea either both antennas of the first 

 pair geniculate or neither does. The genera Cyclops and Canthocampus 

 agree in the character of having a pair of clasping antennas. The 

 marks of separation between the two families to which these genera 

 respectively belong are rather too complicated to be conveniently dis- 

 cussed in this chapter. It may however be noted that, while the two 

 species of Cyclops above named both have the first antennas seventeen- 

 jointed, none of the Arpacticidae have more than ten joints in these 

 appendages ; in Canthocampus stapbylinus they are eight-jointed. 



When allowance is made for the scarcity or absence of any pub- 

 lished information on the subject, the crustacean fauna of this county 

 may now claim not only to have shown excellent promise for future 

 researches, but to have given an earnest of success by already accom- 

 plished discoveries of unusual interest. In adding three species at once 

 to the rather limited number of English terrestrial isopods Bucking- 

 hamshire will not easily be rivalled by other counties. 



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