370 M. F. Plateau on the Freshwater 



the sake of brevity, I shall not reproduce the description of 

 tlie musculature ot the body ; but 1 may indicate one peculia- 

 rity : in the antennie, the pereiojx^ds, and tlie uropods, whilst 

 we see in each moveable joint a tlexor muscle, we always find 

 as its antagonist a large transparent elastic cylinder, without 

 any strise, and presenting here and there a few brilliant nuclei. 

 This is perhaps the very elongated prolongation of a very 

 short muscle. 



Notwithstanding what has been said, Cydopsina castor 

 always swims with the ventral surface downwards. Cantho- 

 ca7nj)tus staph i/h'nus swims with the tail as a continuation of 

 the body, and only elevates it when moving upon the glass 

 plate of the microscope. Natation is effected solely by the 

 antennules, and the pereiopods merely enable the animal to 

 maintain its position in the midst of the liquid. The Copepoda 

 possess a density higher than that of pure water. When 

 recently killed, they fall to the bottom of the liquid at the rate 

 of 5 millims. per second. 



The presence of an optic ganglion for each eye is the only 

 new point that I have ascertained with regard to the nervous 

 system. I have reobserved the curious sleep of the Cyclopidae 

 spoken of by M. Zenker. 



When submitted to the discharge of a Ley den jar of 1 litre 

 capacity, these little animals fall to the bottom of the water as 

 if thunderstruck ; but, singularly enough, in an hour they 

 recover from this stupefaction, and swim about again with 

 vivacity. There is some analogy between these results and 

 those recently obtained by Dr. Richardson, who saw a pigeon 

 and a toad resist the shock of a spark more than 70 centi- 

 metres in length, produced by the colossal induction-coil of 

 the Polytechnic Institution*. 



With regard to the digestive apparatus, I observed on the 

 inner surface of the tunica propria of the first part of the in- 

 testine a layer of enormous, transparent, cylindrical epithelial 

 cells, which probably bear vibratile cilia. I was led to this 

 last supposition by the characteristic rotatory movements un- 

 dergone by particles of alimentary substances in the intestine 

 of a Cydopsina. If my observation were confirmed, it would 

 prove that vibratile cilia may exist in the digestive tube of the 

 Articulata (leaving the Rotatoria out of the question). 



A series of experiments made simultaneously upon Cyclops 

 quadricornis and Daphnia simits, with regard to the influence 

 of sea-water upon these animals, gave me the following re- 

 sults. The Cyclops dies in sea-water in a few minutes ; the 



' Le Cosmos, Oc-tober 23, 1869, p. 44^}. 



