TURBELLARIA. 77 



low, the muscular, the circulatory, and the nervous 

 systems being feebly developed or indistinct. The 

 whole skin seems to be a highly sensitive and 

 delicate organ of touch, and a well -furnished visual 

 apparatus exists in the majority. Two, four, eight, 

 or more specks are visible near the front of the 

 body ; sometimes a varying but considerable 

 number are gathered into two clusters, and in one 

 species the whole fore parts are studded with these 

 specks. These are indubitable eyes, which in 

 some cases ofTer a highly curious and beautiful 

 structure under the microscope.* 



The mouth, which is the only opening of the 

 body in most cases, is sometimes placed near the 

 fore extremity, sometimes at the middle, or even 

 behind the middle of the belly. In the Tribe 

 Dendroccela — represented, so far as I am aware, 

 by only one genus in the British seas, that of 

 Planaria as restricted, — the mouth opens into a 

 large throat, containing a very moveable swallow 

 (pharynx), which can be entirely protruded while 

 the animal is eating, and which, even if separated 

 from the body, still continues for a while to swallow 

 all that is presented to the mouth.f 



Many of the species increase by spontaneous 

 division ; the body dividing across, when each half 

 becomes a perfect animal, and again divides, and 

 sub-divides. But the more highly organized kinds 

 produce eggs, which are developed in a way quite 

 peculiar to themselves. A large single egg, (or 

 egg-like body,) is laid, in which are gradually 

 formed many roundish embryos ; these become 

 flattened and lengthened, and after a while display 



* Oersted, " Plattwurmer," pp. 6, 56. 



t Baer, "UberPlanarien," Nov. Act. Acad. Leop. 1826, p. 716. 



