446 



CHARACTERS OF INVERTEBRATE ANIMALS 



BRAIN 



central nervous system consists of a pair of brain-ganglia in the 

 front part of the body, giving off various nerves, of which the two 

 longest take a backward course. Upon the upper side of the head 

 region are two simple eyes, each of which is provided with a lens 

 and supplied by a special optic nerve. 



The Land Planarians largely agree with the preceding in 

 structure, but are much more worm-like in appearance, though 

 equally unsegmented. Some of the tropical forms attain a large 

 size (up to 1 8 inches when extended), and are brilliantly marked 

 with longitudinal streaks of colour, but our few native species are 

 dull in colour and insignificant in size. An example is Rhyn- 

 chodemus terrestris, which is under an inch in length, whitish in 

 colour below and dark-grey above. 



The commonest example of the marine Turbellaria native to 

 Britain is probably Leptoplana trem.ellaris (fig. 275), which is 

 frequently to be found adhering to the under surface of stones 

 between tide-marks. It belongs to a group which is exclusively 



marine, and differs in a number of 

 important particulars from Planaria 

 lactea. The flattened oval body is 

 rounded at either end but consider- 

 ably broader in front. Not only is 

 the animal able to progress in the 

 gliding manner already described, 

 but it can also swim with consider- 

 able rapidity by flapping the sides 

 of the body up and down. It then 

 looks curiously like a minute Skate. 

 The mouth is situated about the 

 middle of the under surface, and 

 the pharynx, when protruded from 



Fig. ^ni.-L.ptoplana tremellaris (enlarged), as it, reSCmblcS a wide funncl wlth 

 seen from under surface, with pharynx protruded to plaited sideS. This rCp-ioU of the 

 exCenor ^ o 



gut leads into an elongated oval 

 stomach from which branching, blindly-ending tubes radiate in all 

 directions. The bilobed bi^ain is situated fairly far back, and gives 

 origin to a number of radiating nerves. There are four groups 

 of simple eyes situated on the upper side of the body, in the 

 neighbourhood of the brain. 



Certain Turbellarians belonging to the same group as the 



eye: 



PHARYNX^ 



STOMACH 



