450 Our Fresh-Water Planarice. 



nutrient vessels, which, issue from the extremities of the 

 intestinal cceca. Duges says the circulation is best seen in 

 Polycelis nigra, and P. brunnea, and in the marine Leptoplana 

 tremellaris. I confess that notwithstanding the most patient 

 observations, I have not yet been able to make out completely 

 the circulation in these worms ; young specimens, from their 

 comparative transparency, I thought would readily show it, 

 bat I have been disappointed in my examinations.* The 

 circulation is readily enough seen in the equally small species 

 of snail-leeches (Glossophonidce) • why it should be so difficult 

 to make out in the Planarice I cannot tell, but Duges is so 

 trustworthy an authority, that one is not inclined to dispute 

 his statements. The mesial dorsal trunk in P. nigra and 

 P. brunnea is described by him as being tortuous and narrow, 

 sometimes more difficult than at other times to make out, on 

 account, no doubt, he adds, of its dilating and contracting ; the 

 same systole and diastole may also be noticed in the lateral vessels. 

 This arrangement of the circulation in the Planarice reminds 

 one of that of the leeches. 



Of the respiration little need be said. The circulating fluid 

 is oxygenated by the air contained in the water over the 

 whole surface of the body, the numerous minute cilia which 

 clothe the margin of the body, helping to bring fresh currents. 

 Planarice require a constant renewal of water, if kept many 

 together in a vessel, or they soon die.f 



The Planarice exhibit the phenomena of reproducing lost 

 parts, almost emulating, in this respect, the Ht/drce. Sir John 

 Dalyell says of the black Planaria (Polycelis nigra), that "it 

 is privileged to multiply its species in proportion to the violence 

 offered to its otherwise delicate frame. It may be almost called 

 immortal under the edge of the knife. Innumerable sections of 

 the body all become complete and perfect animals. If the head 

 be cut off, a new head replaces it ; if the tail be severed, a new 

 tail is acquired." Duges experimented considerably, and with 

 the same result as Dalyell. An individual cut into eight or ten 

 parts produced the same number of complete animals. The 

 time required to reproduce a lost part depends on the time of 

 year — fourteen or fifteen days being required in winter, four or 

 five in summer. The species also differ in this respect. This 

 reproduction by division appears to take place sometimes 



* The only point of the cireulaiion I have distinctly seen, occurred in a 

 specimen of P. lactea. I noticed various rami fixations of the water-vascular 

 system in the regions of the eyes, under \" objective, but could see no dorsal or 

 lateral channels. 



t They can, however, bear extreme cold without being lulled. I have some 

 specimens of P. torva and Polycelis nigra which have been frozen in a tumbler 

 of water during the last frost. They are now as active as ever. Some of these 

 specimens were not more than two lines long, and about 6ix weeks old. 



