PHYSIOLOGY OF THE INVERTEBRATA. 313 



gradually increase in size until the equator is reached, and 

 from the equator to the ocular plates they again diminish." 

 The nerve-trunk is enveloped by a fibrous sheath containing 



Fig. 57.— Structure of a Nerve-tkunk of Echinus. 

 {After Romanes and Ewart.) 



pigmented cells. The nerve-trunk consists of delicate fibres, 

 and of fusiform cells (Fig. 57). The cells are nucleated, 



" The lateral branches of the nerve-trunk escape along with, 

 and are partly distributed to, the pedicels ; the remainder 

 breaks up into delicate filaments, which radiate from the base 

 of the pedicel under the surface epithelium (Fig. 56, t). 

 "When one of the large branches is traced through the oral floor 

 after sending a branch to the foot, it breaks up into delicate 

 fibres, some of which run towards the bases of the adjacent 

 spines and pedicellaria?, while others run inwards a short 

 distance towards the oral aperture." 



There is also an external plexus situated under the surface 

 epithelium, and extending from the shell to the spines and 

 pedicellarite. " The fibres (Fig. 58) of this plexus closely 

 resemble those of the lateral branches of the trunk ; but 

 generally they are smaller in size, and have a distinct con- 

 nexion with nerve-cells. The cells consist of an oval nucleus 

 and of a layer of protoplasm, which is generally seen to 

 project in two, or sometimes in three, directions — the several 

 processes often uniting with similar processes from adjacent 

 cells, so as to form a fibro-cellular chain or network." 



Komanes and Ewart * have succeeded in tracing the plexus 

 over the surface of the shell between the spines and pedi- 



* Philosophical Transactions, 1881, pt. iii. p. 836. 



