ON THE DIPLOCHORDA. 343 



the base of the long glandular cells, covering the " front ^' of 

 the epistome. 



Laterally and anteriorly, the ganglion gives off a pair of 



large nerves (PI. 24, fig. 13, p. o. 7i.) which pass downwards 



and outwards along the " inner " surface of the epistome, 



where they unite (fig. 12), and passing round the base, they 



meet the broad ring. 



This arrangement, allowing for the peculiar modification of 

 the epistome, is closely similar to that of Phoronis {Actino- 

 t roc ha) and B alanoglossus. 



Lastly, along the mid-ventral surface of the trunk there are 

 several nerve-fibres running longitudinally, and some of these 

 pass into the " pedicle " in a peculiar manner, where they 

 appear as fine nervous cylinders. The detailed description of 

 these and of the pedicle will be given in future work. 



The mid-dorsal and two lateral epistomial branches have 

 their homologues in Balanoglossus and Actinotrocha, 

 whilst the p re-oral nerve-ring of the latter can be compared to 

 the '' front '^ ring of Cephalodiscus, especially when the 

 relationship of both to the ccelomic muscle-strands is con- 

 sidered. One can assume that the condition of the pre-oral 

 nerve-ring of Cephalodiscus has been arrived at by a pro- 

 cess of centralisation by which the ring, and with it the origin 

 of the muscles, has been moved into the centre of the disc, for 

 the formation of a sucker. 



I do not feel justified in giving a complete diagram of the 

 nervous system of Cephalodiscus until I have confirmed 

 certain points not dealt with here. It has been shown that 

 the lateral trunks of Phoronis may be regarded as the two 

 halves of the dorsal nerve of Balanoglossus, in the trunk 

 region, and that their peculiar position can be accounted for 

 by displacement due to reduplication of the trunk. The same 

 reasoning would apply to the case in hand, the lateral nerve 

 trunks of Cephalodiscus occupying the same position as in 

 Phoronis. 



Sense-organs. — The consideration of the curious masses 

 of pigment surrounding the oviducts, which were at first 



