446 Our Fresh-Water Planarice. 



several kinds are found, of which, Planaria lactea, P. torva, 

 Poly cells nigra, P. brunnea are common everywhere in ponds, 

 streams, and ditches. A favourite place of resort of these 

 creatures is within the stems of Sparganium, whence they may 

 be readily picked or scraped off with the point of a knife and 

 placed in a bottle of water. Difficulties of dissection long- pre- 

 vented naturalists assigning to the Planarice their true place 

 in the animal kingdom ; Cuvier, following Miiller, Linnaeus, 

 Pallas, and Laniark, placed them amongst the parenchyma- 

 tous lntestinalia , or Trematocle Entozoa (as the Flukes Distoma), 

 acknowledging at the same time their resemblance to certain 

 species of the Hirudinidce or leech family. The Planarice have 

 affinities with both these families, and lead from one to the 

 other. Let us suppose we have under examination the largest 

 of the British fresh-water Planarice, viz : P. lactea ; this species 

 varies a little in colour, which is either cream, roseate, or quite 

 white ; it is from six to ten lines in length, and about two lines 

 in breadth ; we first notice the delicate arborescent form of 

 the digestive system ; we place the creature on a glass slide 

 and hold it up to the light, in its middle part we see a milk 

 white spot which extends linearly towards the posterior ex- 

 tremity ; by allowing the water gradually to evaporate, the 

 animal shows signs of discomfort, and we observe a long 

 cylindi'ical 'tube to be pushed out from a pore slightly pos- 

 terior to the middle of the body : this pore is the mouth, and 

 the tube is the proboscis, a formidable instrument of attack in 

 these creatures ; Ave notice two black oculiform spots, parallel, 

 and placed on the anterior part of the back ; a little below the 

 oral aperture we see, but very indistinctly in P. lactea, another 

 pore which belongs to the generative system ; we find the body 

 to be slimy, very soft, and readily breaking up if not handled 

 with great care. Let us take another specimen of the same 

 species, and with the aid of a camel's hair pencil, place it 

 gently in a vessel of water, and observe its locomotive powers ; 

 we see it gliding in an even and regular manner, like a Umax, 

 or slug ; if we touch it, the animal twists itself in various folds, 

 or it fixes its head portion to the vessel, and, by contracting, 

 brings the other parts of the body along, then the posterior 

 part is attached, and the head portion elongates and advances. 

 In the species Polycelis nigra, P. brunnea, Planaria torva, we 

 observe the gliding locomotion to be frequently exercised on 

 the surface of the water, the ventral side of the animals being 

 uppermost ; none of the true fresh-water Vlanaricc can be said 

 to swim ; but an allied marine species, Leptoplana tremel Inn's, 

 progresses by Happing its sides as a Ray its fins.* When we 

 consider how varied and energetic are the movements of the 

 *Dugea " Annal. des Sciences, Nat." xv. 151. 



