S8 Pi'uccedlngis of f/ie Roijal Soclrfi/ of Vidorid. 



head, is n single a})citiuc', the comnion ojieuing of the 

 j)iol)Oscis sheath and alimentary canal, or, to speak more 

 exactly, the opening of the rhynchodieum. A little, hehind 

 this ai)erture there is, on each side of the head and 

 somewhat more towards the dorsal than towards the ventral 

 suiface, an irregular group of minute black specks, the eyes. 

 The number of the eyes is probably not constant, ixud the 

 size is certainly very vaiiable ; in one specimen examined 

 there were about twenty in each group. 



On the ventral surface of the head are situated the two 

 very minute ()])enings of the cephalic pits, one on either side 

 of the middle line, but these are only recognisable in sections, 

 though sometimes there appears to be a transverse groove, 

 visible with a pocket lens, in which they probably lie 

 (Fig. 3.) 



The animal crawls normally with an even, gliding motion, 

 much like a Planarian, leaving behind it a slimy track. The 

 motion is probably due in part to muscular and in part to 

 ciliary action, the proboscis being at the time com|)letely with- 

 drawn into the body. If the worm is irritated, however, the 

 proboscis is suddenly shot but from the anterior end with 

 wonderful rapidity. This proboscis is relatively of enormous 

 size, being, even when shot out only to the normal extent, fully 

 as long as the body of the worm, if not longer. After remain- 

 ing out foran instant it is more. slowly withdrawnaud thisever- 

 sion and withdrawal may be repeated several times in rapid 

 succession. Frequently, however (Fig. 1 2), the proboscis; breaks 

 away fiom its attachment round the mouth of the proboscis 

 sheath and remains attached to the body of the worm only by 

 the retractor muscle, which appears as a long, narrow thread 

 coming out from the opening of the ihynchodceum. When 

 detached in this manner the proboscis is actuallj- larger than 

 the body of the animal. The colour of the everted proboscis 

 is pure wdiite and its surface is quite furry from the presence 

 of innumerable little glandular papilke, which secrete a 

 sticky fluid. Under normal conditions the proboscis may 

 probably be everted and withdraw^n again for an indefinite 

 number of times. When fully extended it adheres slightly 

 to the surface on which it falls and hence, as a necessary 

 consequence, when the proboscis is withdrawn again the 

 body of the animal is pulled forwards over it. In this way 

 the animal may progress, using the proboscis as a means of 

 locomotion. In the case of Tetrastemona arjricola von 



