2 



Miss D. J. Lloyd. Influence of Osmotic 



The length of the fully grown specimens obtained at Plymouth lies 

 between 5 and 6 mm., though the worms are found sexually mature from a 

 length of 4 mm. It occurs between tide-marks and near the borders of 

 a small stream. Density determinations made on different occasions of the 

 water in which it was found, show that it is normally exposed to a very wide 

 variation in osmotic pressure. The lowest density value recorded was TOOL 

 i.e. a value corresponding to almost fresh water; the highest was T028. 

 The mean density of the sea-water at Plymouth is l - 023. This fact is an 

 important one to take into consideration when placing the animal under 

 experimental conditions in the laboratory. 



Before proceeding to the main part of the work, it seemed advisable to 

 make three sets of preliminary experiments : the first of these was to 

 determine the limits of osmotic pressure within which whole worms are 

 capable of living ; the second was to observe the morphological changes 

 accompanying starvation in whole animals ; and the third was to make a 

 histological study of the regeneration in sea-water. These are all described 

 in detail below. 



For the experiments in regeneration, only regeneration of the posterior 

 end was considered. 67. ulvce differs from most Triclads in that it will only 

 regenerate a new head in the presence of the cerebral ganglia (6). As it 

 was wished to study the effects of removing large pieces, the experiments 

 were necessarily limited to regeneration of the posterior end. To secure 

 uniformity, the worms were all transected half way down the length of 

 the body. 



67. ulvce, is capable of living for many months in a state of starvation, 

 during 1 which it slowly diminishes in size. Nevertheless, worms kept 

 without food until reduced in length from 5 - 5 to 1*5 mm., when bisected, 

 restored their normal form as rapidly as did fully fed specimens. This 

 point is emphasised here because in the experiments considered in this paper 

 the animals were left entirely without food. This was done, firstly, to avoid 

 contaminating the experimental waters, and, secondly, because worms 

 regenerating in different solutions develop the new mouth at very different 

 intervals after section, and thus the starvation experiments progress equally 

 rapidly and more uniformly. A number of observations were made on the 

 regeneration of 67. ulvce under both starving and fully fed conditions, and the 

 differences which are found consist only in the rather less degree of reduction 

 that occurs when feeding is taking place while the worms regenerate. 



The figures of the whole worms are camera sketches made from living- 

 material. The sections were made from material fixed in corrosive acetic, 

 and stained in picronigrosin and eosin. The sections used for studying the 



