:>. 140 4 



PARA STTIC COPEPODS—CALIQIBM— WILSON. 



517 



Such observation has been made in the three species last named and 

 the respiration was seen to be exactly like that described by Hartog 

 for Cyclops, save that it was not as regular. 



It can be seen to best advantage in late chalimus stages, in which the 

 walls of the abdomen are usually very transparent, while the movement 

 itself seems more vigorous. 



When the rectum contracts during peristaltic movements there is 

 left only a linear cavity along the mid line. Then the dilator muscles 

 immediately pull the rectum out to its full width, at the same time 

 opening the anal valves for an instant. This action fills the rectum 

 with outside water, which 

 then operates by endos- 

 mose through the thin rec- 

 tum wall. 



This same method of res- 

 piration is as normal to the 

 nauplii of these parasitic 

 forms as it is to those of 

 the free-swimming genera, 

 and the author has repeat- 

 ed ly observed it also in the 

 metanauplius of Caligus 

 rapax and in the chalimus 

 of the same species, as well 

 as those of Caligus curtus 

 and Zepeophtheirvs ecl- 

 wardsi. The fact that it 



is thus the only method of respiration throughout the period when the 

 legs are as } T et undeveloped, and that all the muscles concerned in it 

 remain in the mature form, furnishes strong circumstantial evidence 

 that it is the method also in the other forms here described, but in 

 which it has not as yet been actually observed. 



MUSCULAR SYSTEM. 



Copepods belonging to the genera here considered are as a general 

 rule so transparent that their musculature can be determined with very 

 little eifort. And then the muscles are so plainly striated that there 

 is very little danger of mistaking them for any other tissue. Indeed, 

 the striation in the muscles of Caligus curtus were among the very 

 first observed in any animal, and their discovery here by Pickering 

 and Dana was about contemporaneous with that in human muscle by 

 Doctor Hodgkin. (Pickering and Dana, 1838, p. 81, footnote.) 



The frontal plates arc flexed bv two short and slender muscles, situ- 

 at sd in nearly tin 1 same place in all the genera, directly behind the 

 lunules, attached to the posterior portion of the plates, and running 



Fig. 23.— Oblique muscles which operate the rectum in 

 Caligus rapax during respiration. 



