532 



SCIENCE. 



[N. S. Vol. XVII. No. 431. 



is approaelied from tlie other side. It is 

 difficult to explain tlie orientation of Vol- 

 vox on the' theory that it is brought about 

 by differences in the intensity of illumina- 

 tion on the two sides of the organism. Ac- 

 cording to this view, we should expect that 

 as Volvox passes from weak into stronger 

 light its rate of speed would be decreased, 

 but this does not occur. The explanation 

 of orientation in this form is not so simple 

 a matter as it might seem. 



The Blood Flow and the Structure of the 

 Vessels in the Earthworm: J. B. John- 

 ston and Saeah W. Johnson. 

 We have previously reported the results 

 of an experimental study of the course of 

 the blood flow in Lumhricus, which showed 

 that the circulation in this worm is not 

 segmental, but strictly systemic. This view 

 of the circulation opened two lines of fur- 

 ther inquiry: What happens when the 

 hearts are removed from the circulation by 

 cutting off the head segments of the worm ; 

 and what is there in the structure of the 

 blood vessels to determine the course of 

 blood flow? A series of regeneration ex- 

 periments and the study of the histology of 

 the blood vessels have given striking con- 

 firmation of our previous conclusions. 



1. In all animals from which the head 

 segments were removed there was an_ enor- 

 mous collection of blood in the anterior end 

 of the worm, including the regenerated seg- 

 ments. Such a condition woiild probably 

 not be brought about if there were a seg- 

 mental circulation in the normal worm. 

 Usually all circular vessels were crowded, 

 but the intestinal vessels and spaces were 

 more distended than the parietal vessels. 



2. The dorsal vessel and all the vessels 

 connected with it are provided with valves 

 which determine the direction of the blood 

 flow. In the dorsal vessel at the level of 

 each septum is a pair of large, thick, flap- 

 like valves, one attached to either lateral 



waU. of the vessel. These valves open 

 forward and are closed at the time of each 

 contraction-wave. The parietal, dorso- 

 intestinal and dorso-typhlosolar vessels are 

 each provided with similar valves, so placed 

 in the mouth of each vessel that the blood 

 can flow freely into the dorsal vessel, while 

 each vessel is closed by its valves in ad- 

 vance of the contraction-wave of the dorsal 

 vessel. No valves have been found in any 

 other vessels, but these are enough to direct 

 the blood flow. 



3. The walls of the vessels are made up 

 of three coats: (a) A layer of extremely 

 thin, flat, endothelial cells; (&) a connec- 

 tive tissue membrane containing longi- 

 tudinal fibers, probably muscular; (c) a 

 layer of circular muscle fibers. The valves 

 are masses of cells connected with the 

 connective tissue layer. The circular 

 muscle layer is especially thickened at the 

 valves in the dorsal and parietal vessels, 

 and the contraction of these bands of mus- 

 cle presses the valves together, completely 

 closing the vessels. A similar mechanism 

 in the intestinal and typhlosolar vessels 

 has not been seen, but the valves are so 

 placed as to open toward the dorsal vessel 

 and tb be closed by backward pressure. 



On Phyllodistomum aniericanum n. sp., a 

 parasite in the Urinary Bladder of 

 Aniblystoma tigrinum Green, in Min- 

 nesota: Heney Leslie Osboen, Hamline 

 University. 



This genus, recently founded by Braun,* 

 has been reported from central Europe, 

 eastern Asia and northeastern Africa, from 

 the urinary bladders of fish and amphibia, f 

 but has not hitherto been recognized in this 

 hemisphere. I have found that flukes gen- 

 erically identical with the old world ones, 

 • ' Ueber Clinostomum,' Zool. Anzeig., XXII., 

 pp. 484-488, 1900. 



t Looss, 94, Distom. Fiseh. u. Frosch; Sturgis, 

 97, Zool. Bulletin, I., p. 57; Odhner, '00, Cent. 

 F. Bakt. u. Parasit., XXXI., pp. 58-69. 



