CHAP. IX. DEVELOPMENT OF COPEPODA. 87 



rudiments of the third and fourth pairs of natatory feet 

 have made their appearance in the form of cushions 

 fringed with setae, and the body consists of the oval 

 cephalothorax, the second, third, and fourth thoracic 

 segments, and an elongated terminal joint. In the 

 Cyclopida3 the posterior antennae have- lost their 

 secondary branch, and the mandibles have completely 

 thrown off the previously existing natatory feet, whilst 

 in the other families these appendages persist, more or 

 less altered. " Beyond this stage of free development, 

 many forms of the parasitic Copepoda, such as Lernan- 

 thropus and Chondracanthus, do not pass, as they 

 do not acquire the third and fourth pairs of limbs, nor 

 does a separation of the fifth thoracic segment from the 

 abdomen take place ; others (Achtheres) even fall to a 

 lower grade by the subsequent loss of the two pairs of 

 natatory feet. But all free Copepoda, and most of the 

 parasitic Crustacea, pass through a longer or shorter 

 series of stages of development, in which the limbs 

 acquire a higher degree of division into joints in con- 

 tinuous sequence, the posterior pairs of feet are de- 

 veloped, and the last thoracic segment and the different 

 abdominal segments are successively separated from 

 the common terminal portion." 



There is only one thing more to be indicated in the 

 developmental history of the parasitic Crustacea, namely 

 that some of them, such as Achtheres percarum, certainly 

 quit the egg like the rest in a Nauplius-like form, inas- 

 much as the plump, oval, astomatous body bears two 

 pairs of simple rowing feet, and behind these, as traces 



