206 



INVEETEBEATA 



CHAP. 



has been worked out by H. Wilsou (1911). The adult lives on the 

 gills of the rock-bass, Ambloplites rupestris : it is a sac-like organism 

 fixed by two conjoined arms to the host; it shows no trace of 

 Copepod structure except the long egg-tubes, which the female bears 



protruding from the end of her 

 body. If we were to classify by 

 adult structure alone, no one 

 would dream of regarding Ae- 

 ther es as a Copepod; but yet 

 every zoologist is fully convinced 

 that Adheres is a modified 

 Copepod that is to say, that it 

 is descended from an ancestor 

 which was like Cyclops or Calanus 

 or some other typical Copepod 



genus. 



FIG. 152. Dorsal and lateral views of just- 

 fixed female of Adheres ambloma/it. 

 (After Wilson.) 



A, dorsal view. B, lateral view. Letters as in 

 preceding figure ; fr.f, frontal filament. 



Now the Nauplius and Meta- 

 nauplius stages are completed 

 inside the egg membrane, and 

 the young animal hatches out as 

 what is termed a Copepodid 

 namely, in a form wliich every 

 one would recognize at a glance 

 as showing the typical structure 



of a Copepod, that is, of the ancestor. When, however, we look 



closely at this Copepodid larva we find that it differs from an ordinary 



Copepod in the following points: (1) There are but two free 



segments in the thorax each carrying 



a pair of forked swimming appen- 

 dages, whereas five such segments on 



the normal Copepod carry four pairs 



of forked swimming appendages and 



one rudimentary pair ; (2) the exo- 



podites and endopodites of these legs 



are not divided into joints, while the 



corresponding members in an ordinary 



Copepod are many-jointed ; (3) the 



first antennae are short, stumpy, and 



lew-jointed, as contrasted with those 



in an ordinary Copepod, where they 



are normally long and Composed of FKJ. 153. Lateral view of female 



many joints ; (4) the second antennae Adheres amUopiitis after adult 

 in the Copepodid are likewise exceed- (After walS ) have *"" *^ A ' 

 ingly short, and although forked each Letters as in ' preceding flgures _ 



branch is unjomted and the inner 



one terminates in a hook, whereas in the normal Copepod this 

 hook -like termination is not found; (5) in the jaws, i.e. mandible, 

 maxillae, and maxillipede, there is nothing which'could be described 



