356 Miss D. J. Lloyd. Influence of the Position of the 



obligations to the Koyal Society, the Zoological Society, and the University 

 of Cambridge for the use of their tables at the Plymouth Laboratory. 



II. Material. 



The experiments were performed on members of the species Gunda ulvce. 

 G. ulvce is a small Triclad Planarian belonging, with other marine planaria, 

 to the order Maricola, and to the family Gundidae (Procerodidse). Planaria, 

 the genus on which most of the work on Triclads has been done, is a fresh- 

 water form (order Paludicola). 



The structure of G. ttlvce is quite typical of all Triclads and has been 

 described by Wendt (8) and Iijima(5). It contains the usual trifid gut, which 

 opens at the point where the three branches meet into a protrusible pharynx. 

 The mouth is situated near the hind end, rather more than three-fourths 

 down the length of the body. The accessory genital organs lie behind the 

 mouth. The cerebral ganglia are about one-fifth the body length from the 

 anterior end. Two nerve cords run backwards from the ganglia, joining in 

 the tail to form a complete ring. The two eyes lie in front of the ganglia, 

 and at the front of the head are the so-called auricular processes, two in 

 number. 



•III. Experiments. 



A. Posterior Regeneration. 



G. ulvce will regenerate tails completely either in the presence or absence 

 of the cerebral ganglia. Text-fig. I, A, shows the levels at which cuts were 

 made. Fragments taken for posterior regeneration were head-pieces ADD, 

 AEE, AFF ; middle pieces, BBFF and DDFF. Figs. 1-3 show the progress 

 of tail regeneration in a worm cut through the middle of the pharynx region 

 (AFF). For worms cut off in front of the pharynx (AEE, I, 4), it is 

 essentially similar, except that the newly regulated worm is proportionally 

 smaller. The figures, which are drawn to scale, show quite clearly that the 

 new tail is produced by a differentiation of the old tissue. 



The next group of text- figures shows the regulation of fragments taken 

 from the middle of the worm. The characteristic difference in behaviour 

 hetween the pieces with and without cerebral ganglia (figs. 5-7 and 8-10 

 respectively) is quite apparent. The former show regeneration taking place 

 at both ends of the worm, finally resulting in complete restoration of both 

 head and tail. The formation of tissue at the head end is seen to check the 

 rate of growth at the tail end. In fragments without ganglia (figs. 8-10) 

 no head is regenerated and restoration of the tail proceeds as rapidly as in 

 the head-pieces (with ganglia) described first. Eestoration of posterior parts 



