PLATYHELMIA. 35 



those organs which have more especially to do with irritability 

 and spontaneity. 



Around the pharynx is placed a nerve-ring, the plane of which 

 slopes downwards and backwards. In its dorsal part there is a 

 swelling, the cerebral ganglion (c.g) } on either side, and a similar 

 unpaired ventral ganglion (v.g) below. From each cerebral gang- 

 lion two small filaments, nerves, are given off to the head-papilla, 

 in which they branch, whilst a stouter lateral nerve (Ln) runs 

 back almost to the end of the body, giving off branches as it 

 does so. It lies just within the ventral body-wall, below the 

 reproductive organs, and is separated from its fellow by about 

 one-third the breadth of the body. Excessively fine filaments 

 are given off from the ventral ganglion to surrounding parts. 



Histology. Two elements make up the essential part of the 

 nervous system (a) Nerve-cells (ganglion-cells), (b) Nerve-fibres. 

 The nerve-cells are most numerous in the ganglia, but are also 

 present in less abundance in parts of the nerve-ring. They are 

 irregular in shape, with several projections or processes ; their 

 protoplasm is clear and they contain a very distinct refractive 

 nucleus. The nerve-fibres are extremely delicate threads, which 

 make up the nerves and most of the nerve-ring. They are con- 

 tinuous on the one hand with the processes of the nerve-cells, on 

 the other with the various organs of the body. 



The nerve-cells are connected with one another by means of 



their processes. 







DEVELOPMENT (Fig. 8). 



The life-history of the Liver-fluke, like that of many other 

 parasites, exemplifies alternation of generations, a phenomenon 

 far less general in the case of animals than in the case of plants. 



From the egg a young fluke does not proceed direct, but a 

 number of asexual stages i.e., forms capable of asexual multipli- 

 cation, intervene between it and the sexually mature adult. The 

 sequence of events is as follows: (1) A ciliated embryo escapes 

 from the egg and becomes parasitic within the lung-chamber of 

 a water-snail, there degenerating into (2) a shapeless sac or sporo- 

 cyst. (3) Within the sporocyst numerous cylindrical red-m are 

 produced by a process of internal budding, and these feed upon 

 the liver of the snail. There are usually several generations of 



