No. 3.] STUDIES ON THE HETERONEMERTINI. 405 



genetic, relationships of the four types of nemertean ganglion 

 cells: jj 



Sensory \ ^ ttt/' ( Motor 



cell S ^ivi '^^^^^• 



It may be noted that in the nemerteans the size of the 

 ganglion cells stands, as a rule, in proportion to the size of 

 the body. Thus Cerebratubis lacteus, the largest species exam- 

 ined here, has larger cells than has Linens sp.; and the latter 

 species has larger cells than the still smaller L. gesserensis. 

 And I find in the Metanemertini also that Amphiporus glutinosus 

 and A. virescens have larger cells than the smaller species, 

 Tetrastemma catemilattim, .T. verjnicuhmi, and Siichosietnma 

 eilhardi. An exception to this rule is Carinella annulata, with 

 smaller cells than Liiieus sp.; but Carinella is a most primitive 

 form, in which the ganglion cells and the whole nervous system 

 have become but little differentiated. 



III. Nerve Tubules. 

 A. Cell III. 



In Linens gesserensis I have studied the nerve tubules 

 on thin sections ic. 3 /Li thickness) on material prepared with 

 sublimate (aqueous, alcoholic solutions), Flemming's and Her- 

 mann's fixatives, and treated with different stains. 



Each ganglion cell has one axis-cylinder process, and I have 

 been unable to find dendritic (so-called " protoplasmic," or 

 " Deiter's ") processes. 



The cytoplasm of the ganglion cell (Figs. 3-14) is usually 

 most vacuolar distally, where the cell tapers towards the axis- 

 cylinder pole. But even here the mass of cytoplasm is always 

 bounded by a peripheral, fine-grained layer {Alv). This periph- 

 eral layer is directly prolonged along the axis cylinder, and pro- 

 duces the very fine spongioplasmic sheath of the latter (Figs. 

 3-14, Alv). This sheath is, with the exception of the sheath 

 of Schwann (to be described later, cf. "Neuroglia"), the only 

 staining portion of the nerve tubule, and also the only part in 

 which a fine-grained structure can be discerned. This fine 



