220 CLASS VII. 



occasion to the name, was first, as it seems, observed by DUGE*S in 

 Planarice, although he did not refer it to cilia (Ann. des Sc. nat. 

 Tom. xv. p. 165), whilst VON BAER observed at the same time an- 

 other phenomenon which coul'd only be an effect of these cilia, 

 that when a portion of these animals is cut off it continues to rotate 

 circularly in the water (Nov. Act. Acad. Ccesar. Leop. Carol. Tom. 

 xiii. P. 2, p. 711). 



The internal structure of these creatures was first recognised with 

 precision in this century, especially through the investigations of 

 DUGES, Y. BAER, FOKKE, OERSTED, and QUATREFAGES; it presents 

 important differences in different genera whilst the external habitus 

 is similar. We must confine ourselves to the notice of a few general 

 traits. 



Besides the cilia on the external surface, the external integument 

 in many is distinguished by the presence of cells with nettle-threads, 

 like as we stated in Acalephae. Beneath the integument there is 

 a layer of transparent, homogeneous tissue, which, according to 

 QUATREFAGES, supplies, as it seems, the place of muscle, and by its 

 contraction effects the movement of the body. The motion occurs 

 by swimming in the water, the lateral margins beating to and fro, 

 by creeping with bending and straightening of the body, or by equably 

 gliding, much as the gasteropod molluscs move with their so-named 

 foot. 



The oral aperture is sometimes more forward, sometimes more in 

 the middle of the body on the ventral surface. The intestinal canal 

 is in some straight, and extends itself, when the mouth is not placed 

 quite forward, anteriorly as well as posteriorly, with blind termina- 

 tions in both directions. In others the intestinal canal is like a 

 tree divided into branches j in our fresh- water species one branch is 

 seen to run forwards, on both sides provided with blind appendages, 

 and two stems or main branches backwards at the sides of the body 

 (in Planaria lactea the branches may be readily distinguished 

 externally by their dark colour). In other species from sea- water 

 the division of the branches is somewhat different ; sometimes quite 

 retifonn. In the cavity of the mouth is situated a part that can 

 be extended by eversion, serving for the seizure of food, and various 

 in form. It is able, when severed from the living creature, to move 

 independently for some time whilst it swallows greedily surrounding 

 substances which are seen to pass out by the posterior open ex- 

 tremity as through a funnel. With respect to the vascular system 

 little is known ; that which is described as such by some writers, 



