No. 3.] STUDIES ON THE HETERONEMERTINI. 387 



sent alveolar layers (in the sense of Biitschli, '94), will be con- 

 sidered more in detail in treating of the ganglion cells III. In 

 their distribution the cells II are limited to the ventral brain 

 lobe and to the lateral chords, forming the greater part of the 

 ganglion-cell layer of the latter. In their arrangement a radial 

 grouping of a number of cells (thus giving rise to a cell cluster) 

 around an opening in the inner neurilemma seems to be the 

 rule; but the radial clusters are not as distinct, nor is there a 

 symmetrical grouping of them in the lateral chords, as in Cere- 

 bratidus. Further, in Linens 11 is not as distinct from III as 

 in Cerebrattilus, but these two types of cells closely approximate 

 in appearance, so that a very small III may have a marked 

 resemblance to a large II; but the form and structure of the 

 nucleus usually serve to distinguish them. 



In Cerebratuhis the cells II (Fig. 18) differ quite noticeably 

 from those of Linetis, in addition to their greater size. Thus 

 the cytoplasm is usually denser, i.e., there is a proportionately 

 greater amount of spongioplasm, and a coarsely vacuolar struc- 

 ture is seldom found. The nucleus is usually oval and varies 

 considerably in size; it contains a number of irregular chromatin 

 masses, and one (sometimes two) larger, spherical nucleoli («). 

 The grouping of these cells into radial clusters, which Burger 

 ('90b) has shown to be characteristic for them, and which is 

 much more pronounced in this genus than in Linens, is due 

 to the fact that the nerve tubules of a considerable number 

 (approximately fifteen or more) of neighboring cells converge 

 together and penetrate in a bundle through a single opening 

 of the inner neurilemma to reach the fibrous core (Fig. 18). 

 On a section, by which only a few of the cells of a cluster are 

 cut, the arrangement is fan-shaped (Fig. 25, CI. II). This 

 grouping is especially characteristic for II, though sometimes 

 cells I present this arrangement to a limited extent. 



These cells occur only in the ventral brain lobes and in the 

 lateral chords, as shown by Burger ('90b). But their symmetri- 

 cal arrangement in the lateral chords in Cerebratulus, a point 

 hitherto unnoticed, is worthy of mention. In each lateral nerve 

 chord, namely, the radial cell clusters occur as in the ventral 

 lobe, but with a certain regularity of distribution which is not 



