206 



INVERTEBEATA 



CHAP. 



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

 gills of the rock-bass, AmUopUtes 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 Ac- 

 theres as a Copepod; hut 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 Gopepodid — 

 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 Gopepodid 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 

 many joints ; (4) the second antennae 

 in the Gopepodid are likewise exceed- 

 ingly short, and although forked each 

 branch is unjointed and the inner 

 one terminates in a hook, whereas 

 hook - like termination is not found ; 



Fig. 152. — Dorsal and lateral views of just- 

 fixed female of Adheres amUopUtis. 

 (After Wilson.) 



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

 lireceding tigure ; /?■./, frontal filament. 



af^ 



mxp 



Fig. 153. — Lateral view of female 

 Adheres amiloplitis after adult 

 characteristics have been attained. 

 (After VS^ilson.) 



Letters as in preceding figures. 



in the normal Copepod this 

 (5) in the jaws, i.e. mandible. 



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



