Part II. — StichodactylincB and Zoanthece. 145 



stomodpeum 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. xi., fig. 2). I have 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 difJorent regions in each are 

 marked out by very distinct histological characters. 



In Phymantlius, as in practically all comj^ound 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 zooxanthellse 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-staining, 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 jjersists 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 mesogloeal axis 

 a fibrillar reticulation is developed, and extends nearly to the periphery of the 

 layer, but there becomes more like the ordinary endoderm in charactei-, possessing 

 nuclei, gland cells, and, usually, zooxanthellse. 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 

 bx'oader than the mesenterial mesogloea 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. PI. XI., fig. 2 and PI. 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, resi^ectively. It seems desirable, 

 however, to distinguish more closely the various regions exhibiting a different 

 histological structure, and presumably performing a different function. The term 



