21 



flowing posteriorly, and the others into the currents 

 passing to the base of the month. 



The main ouirents are easily seen by plaeing the living- 

 animal on its back, in a drop of sea water on a slide then 

 covering with a thin cover glass and examining with a 

 }in. objective. 



The blood currents described above do not continue to 

 flow for any length of time in the one direction. At one 

 period they may l)e flowing as indicated by the arrows in 

 Plate II., fig. 2. Then they suddenly slacken and reverse, 

 and stream for a time in exactly the opposite course. 

 Sometimes the blood corpuscles are seen to simply oscillate 

 backwards and forwards, making no advance, but at other 

 times they pass rapidly along in a definite manner. 



There are no independent organs of respiration. It has 

 been suggested by Hartog and others that the blood is 

 [•robablv aerated from the sea water contained in the thin- 

 walled alimentary canal by the method of " anal respira- 

 tion,'' which has been described in Ci/do]).-;, Caligiix, 

 Argidna, Daphnia, Cypris and other lower Crustacea. 



The cuticular exoskeleton over the surface of the body 

 i^ in most places so thick that the respiratory change of 

 gases may be supposed to take place much more readily 

 through the very thin layer of chitin which lines the 

 rectum. There are dilator muscles attached to the wall 

 close to the anus, and the peristaltic movements oi the 

 whole alimentary canal may aid in the production of 

 inhalent and exhalent currents of water. It appears, 

 however, to the present author that further precise obser- 

 vations are required to substantiate this hypothesis. 



No organ corresponding to the " shell gland '' described 

 in various lower criistacea, and shown by Claus, Hartog 

 ■,ind others to be a renal organ, has been found, 



