Prof. Ehrenberg^s Researches on the Infusoria. 223 



nearly twice the caliber of any of its tributary transverse 

 twigs. 



It will be observed, that the disposition of this main dorsal 

 trunk, with its collateral branches, is almost exactly that of the 

 vascular system of the Ascidia, so beautifully demonstrated by 

 M. Savigny, which is a strong argument for their being of the 

 same character. No motion of an internal fluid is discernible in 

 their interior, nor has any pulsation, analogous to a heart, been 

 ever observed. Both these phenomena, which would decide the 

 question as to their true nature, Corti asserted that he had ob- 

 served in the Rotatoria and Brachionus, but he was deceived by 

 the tremulous motion of the canal, leading from the mouth to 

 the oesophagus. The same was the case with Gruithuysen, 

 who mistook the motion of the intestine in the Faramacium Au- 

 relia for that of a sap-like fluid. It is worthy of note, that these 

 white striae are attached to the internal, not to the external tunic 

 of the integuments. 



5. Nervous System *. — This name is given to a series of six 

 or seven round glandular-looking greyish bodies, which enve- 

 lope the upper or dorsal part of the oesophagus of the Hydaiina. 

 They are closely connected together, and are distinguished from 

 all the other viscera of the body by their darker tint. The up- 

 permost of these bodies (ganglia), or that situate in the mesial 

 place, is much larger than the rest, and gives ofi", from its apex, 

 a slender branch which proceeds upwards towards the integu- 

 ments at the back of the neck a little before the second pair 

 of vascular twigs, where it forms a slight enlargement (gan- 

 glion) ; it does not stop here, but returns back and unites 

 again, not with the large ganglion from which it was originally 

 given ofl\, but in one of the adjacent smaller ones. A complete 

 cu-cle is thus formed, bearing some resemblance to the nervous 



" According to all our ideas of known physiological laws, the existence of 

 active voluntary motion presupposes the necessity of an animating nervous 

 system. Hitherto, however, no attempt had ever been made to prove its ex- 

 istence. But here again in these animals, excluded by their delicacy and 

 minuteness from the ordinary means of anatomical investigation, the trans- 

 parency of their tissues, as it has enabled us to discover the existence of a 

 muscular, has no less assisted us in the more than probable discovery of its 

 necessary appendage, a nervous tissue. 



