INTRODUCTION. XXV11 



cilia, and gradually pass into the intestine. Where a 

 gizzard is present, the food is crushed in its passage 

 through it and delivered half prepared to the stomach*. 



NERVOUS SYSTEM. This is present under a very simple 

 and rudimentary form. It consists (amongst the marine 

 forms) of a single roundish or oval ganglion, placed against 

 the wall of the oesophagus, towards its upper extremity, 

 on the anal side (Woodcut, fig. i. g), from which sets of 

 nerves are given off in different directions. The latter it 

 is exceedingly difficult to follow ; and in many cases they 

 elude observation altogether. Allman, however, succeeded 

 in detecting, in some freshwater species, a nerve-trunk 

 running through the lophophore, and sending off fila- 

 ments towards the tentacles, and also a filament passing 

 off from the nerve-centre to the oesophagus. The details 

 of the nervous system amongst the Phylactol&mata have 

 been more thoroughly worked out by Hyatt and Nitsche, 

 who have confirmed Dumortier's observation of the union 

 of two ganglionic centres in the nerve-mass in this divi- 

 sion. These are united by a commissure, and each of 

 them supplies the nerves to its own side of the body. 

 Nitsche has also demonstrated the existence of a narrow 

 collar encircling the O3sophagus. Hyatt describes, with 

 some minuteness, the distribution of the nerve-trunks 

 and branches which supply the lophophore and probably 

 the tentacles (though no filaments are to be traced into 

 them), the tentacular sheath, and the various portions of 

 the alimentary canal. The more delicate nervous fila- 

 ments he was unable to define. 



* I have seen the food, after passing through the gizzard, driven back 

 into this organ by the contractions of the stomach, and again submitted to 

 its action. 



