Part IT.—Stichodactyline and Zoanthee. 145 
stomodzeum the mesenterial filaments are trilobed in transverse section, the 
middle lobe bearing the glandular streak or Nesseldriisenstreif, and each lateral lobe 
a ciliated streak or Flimmerstreif (PI. xz, fig. 2). Ihave elsewhere (1898, p. 644) 
suggested that it is scarcely correct to regard the term middle lobe as synonymous 
with the two first terms, nor the lateral lobes as synonymous with the second terms. 
The lobes are very complex in their structure, and different regions in each are 
marked out by very distinct histological characters. 
In Phymanthus, as in practically all compound filaments, the apical region of 
the middle lobe can be sharply distinguished histologically from its lateral regions. 
The apex stains more deeply, owing to the presence of numerous ciliated supporting 
cells with oval nuclei, and usually contains intermingled granular gland cells and 
nematocysts. Beyond this the histological elements are not so closely aggregated ; 
the cell nuclei are rounded, the ciliated layer is not so strong, nematocysts are 
absent, and zooxanthellz usually occur. With very little alteration this tissue is 
continued for a short distance on to the lateral lobes, where it becomes replaced 
by another, which is at once distinguished from all other structures of the Actinian 
polyp by the brightly-staiming, homogeneous character of its cell constitutents. 
Both in sections and in macerations, these are seen to consist of ciliated supporting 
cells, the oval nucleus being arranged at different heights in the various cells, so 
as to produce a nucleated zone, extending practically across the thickness of the 
layer; neither gland cells nor nematocysts are ever mingled with the supporting 
cells, and the ciliation usually persists in preserved specimens, even when absent 
from all other regions of the polyp. 
The hinder region of the filament, where it is connected with the mesentery, 
displays still another histological modification. From the lateral mesoglceal axis 
a fibrillar reticulation is developed, and extends nearly to the periphery of the 
layer, but there becomes more like the ordinary endoderm in character, possessing 
nuclei, gland cells, and, usually, zooxanthellae. As this passes on to the mesentery, 
it becomes indistinguishable from the mesenterial endoderm. The mesogloea of 
the axes supporting the middle and lateral lobes also becomes modified. It is 
broader than the mesenterial mesoglcea immediately behind, and oval or stellate 
cells are numerously developed, so that the whole structure stains very deeply, 
and stands out very prominently from the surrounding tissue. Similar details are 
afforded by the filaments of nearly all Actiniaria which possess a trifid mesogloeal 
axis (cf. Pl. x1., fig. 2 and Pl. xv., fig. 4, the latter a zoanthid). 
It is usual to recognize in the trilobed mesenterial filament only the glandular 
streak and the ciliated streaks, and these terms have been generally employed as 
synonymous with the middle and lateral lobes, respectively. It seems desirable, 
however, to distinguish more closely the various regions exhibiting a different 
histological structure, and presumably performing a different function. The term 
