206 



INVERTEBRATA 



CHAP. 



FIG. 152. Dorsal and lateral views of just- 

 fixed female of Adheres ambloplitis. 

 (After Wilson.) 



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

 preceding figure ; fr.f, frontal filament. 



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

 gills of the rock-bass, Ambloplites rupestris : it is a sac-bike 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 Ac- 

 theres 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. 



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 which 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 

 few-jointed, as contrasted with those 

 in an ordinary Copepod, where they 

 are normally long and composed of 

 many joints ; (4) the second antennae 

 in the Copepodid are likewise exceed- 

 ingly short, and although forked each 

 branch is unjointed and the inner 

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

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

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



FIG. 153. Lateral view of female 

 Adheres ambloplitis after adult 

 characteristics have been attained. 

 (After Wilson.) 



Letters as in preceding figures. 



