50 'Journal of Comparative Neurology and Psychology. 



separated by the abdominal ganglia from which connectives pass to 

 them; (4) a pair of pleural ganglia, not joined by a commissure and 

 not giving off nerves. They are united by means of connectives 

 to the pedal, visceral, and cerebral ganglia of the same side; (5) a 

 pair of cerebral ganglia with their supra-cesophageal commissure 

 and connectives to the pleural, pedal, and buccal ganglia; (6) a 

 pair of buccal ganglia, with a commissure under the oesophagus 

 posterior to its connection with the sac of the radula." (Quoted 

 from Henchman '90, p. 193.) 



A comparison of this drawing with those of pond snails by 

 Lacaze-Duthiers shows a number of differences in respect to the 

 origin of the nerves and the announcement of two nerves that are 

 not shown in his figures. 



III. LYMPH CANALS. 



Structures known as lymph canals we differentiate from vacu- 

 oles, although both have a similar appearance in the fixed cell. 

 This distinction is made after a study of the living nerve cell. 

 In a subsequent section on vacuoles it is suggested that in certain 

 instances the lymph canal, trophospongium, etc., are not real 

 lymph spaces, but isolated and independent vacuoles. That 

 lymph canals do really exist in nerve cells seems to be w^ell estab- 

 lished, as the accompanying review indicates. Our study of 

 fixed material in Helix and Aplysia show^s that the outer border 

 of the cytoplasm is frequently penetrated by spaces, as well as 

 numerous processes from the neuroglia. Many of the drawings 

 of Rhode and Holmgren indicate a similar state of the cytoplasm 

 so that we believe that these lymph canals have a rather general 

 distribution in invertebrate nerve cells. Holmgren in his sev- 

 eral papers has given an elaborate account of lymph-spaces. Ap- 

 parently the same class of structures had been previously described 

 under the caption "intercellular neuroglia" by Rhode. Rhode 

 observed these structures in various animal classes, making a 

 special study of Aplysia, Helix, and Doris. His results are inter- 

 preted in terms of his theory of work on the part of the neuroglia 

 cells. The neuroglia cells are not considered as intruders but 

 as cells which by their activity build up the nerve cell. 



In order to give some conception of the extent and importance 

 of the work on lymph canals, the following rather full review is 

 made. 



