26 PROF. M. M. HARTOGr ON THE 



Cojiepods, Daphnia, Polyphemids, and Leptodora, he finds a " powerful activity of tliese 

 muscles, a rapid rhythmical expausiou and contraction of the rectum, when the animal 

 having been exposed for some time to the pressure of the cover-glass is deprived of its 

 normal respiratory relations, or when the animal in its proper conditions has to expel a 

 bolus of faeces. This shows," he continues, " that the action of the dilators is est-cntial 

 for the expulsion of the faeces, in contradistinction to Weismann's rather teleological view 

 that the joeristaltic motions would certainly suffice for the evacuation of the contents of 

 the intestine. While the former relation, easily verified, shows that in abnormal con- 

 ditions, perhaps of asphyxia, an increased activity of the dilators is excited, which 

 jjerhaps [italicized in the original] may to some extent expose a new surface for respi- 

 ration"*. There are two objections to Claus's view of the matter — one of fact, the 

 other of interpretation. As regards the fact, three stages may be seen on examining a 

 Cyclops under the cover-glass. In the first, with moderate pressure, or none at all, the 

 rhythmic contractions are perfectly regular ; next, with increased pressure, they diminish, 

 become irregular or stop ; last, under yet stronger pressure, and especially when the animal 

 is somewhat crushed, they become again very strong, spasmodic, giving tlie look of 

 cramp. In Dapjhnia, the rhythmic contractions are best seen when the animal is quite 

 free in a zoophyte-trough, with room and to spare to swim and to turn over ; and if it has not 

 been observed before in these Crustacea swimming freely, it is because any observer who did 

 not wdsh to make out a sj)ecial point would be sure to confine his specimen, to save liis eyes 

 and patience. Under similar conditions of free room I have observed this action of the 

 rectum in three groups of Copepoda Natantia, in Caligus and Argulus among the 

 Parasitica, Daphnia, Chydorus, Ilacroccrcus, and Molna, and in Apus larvae in the 

 Phyllopoda, in Gammarus and Aselltis of the Arthrostraca, and in the Zo^ea-larvse of 

 Brachyura and Macrura. It occurs in Cypris and Candona, and is, I have no doubt, 

 universal and normal. 



Next, as regards argument. So many animals lack dilators for the anus, that it is 

 impossil)le to regard them as essential to defecation. The peristaltic action which can 

 urge the faeces down against the friction of the rectal wall is surely sufficient to expel 

 them into the external medium, and the mere presence of the muscles accounts for their 

 incidental employment ; for how could the faeces pass without opening the valves ? 

 Moreover, the greater number of muscular slips serve rather to dilate the cavity of the 

 rectum than to open the valves. 



It is a noteworthy fact, and altogether against Claus's view, that the rhythmic action f is 

 interrupted for some considerable time just when the lower part of the intestine is filled 

 with faeces ; and comes into play with increased activity immediately on their expulsion, 

 just as a man will pant after holding liis breath for a short time. Whatever be the case 

 as regards Cyclop)s breathing by the surface of the body, and especially by the pleura, 

 it is hard to see how such a respiration would suffice for a thick-skinned animal like 

 Caligus, where the cuticle is dense everywhere and separated by a thick hypoderm from 

 i\w ccelomic liquid. This is a far more serious obstacle to respiration than the thin walls 

 of the rectum chitinized though thev be. 



*t5^ 



* Strangely enough, this was published just a year before my first note " On the Anal Piespiratiou of Ci/doj'S,^ 

 though I only saw the pnper recently. 



t i>ce my further remarks on anal respiration in treating of the relitiou of Copepoda. 



