AFFINITIES OF ECHIURUS UNICINXTrS. , 79. 



to liold itself in its tunnel. The setae have a significantly abundant supply of powerful 

 muscles (fig. 12), and the wall of the posterior end of the body is mucli thicker than 

 elsewhere, on account of the comparatively disproportionate development of the layer of 

 longitudinal muscles (PL 7. fig. 9j. 



Setce. — Tliese agree on the whole with Spengel's description of those in Echinrns 

 Pallasii. TJiere is, however, some slight difference observable between the setsc of the 

 anterior pair (PI. 7. fig. 3) and of tlie posterior circlet ; the former ai-e large and very 

 markedlv recurved, while the latter are smaller and liner, with but a slisrht curvature. 

 The anterior seta? project into tlie body-cavity vei-y conspicuously, and possess numerous 

 strong radiating muscles (PL 7. figs. 5, 12; PL 10. fig. l-O). In minute structure each 

 seta presents a pointed, somewhat flattened free end ; about a third of its length back 

 there is a circular constriction, after wliich the seta widens out again to its blunt 

 extremity, which lies embedded in muscle and a protoplasmic mass from which it 

 originated, and in which young setae can 1)3 semi. The chitin of whicli tlic liook is 

 composed exhibits a transverse striation or annulation, less marked at the pointed end. 

 GreeflP says there is a fine canal in the suljstance of the seta at the free end running 

 a sliort way back ; this does not appear to be a canal but to be due to striation, as is so 

 often seen in the ordinary setae of Chajtopoda. 



Bodij-waJl. — The body-wall varies in thickness in ditfereut regions, being thinnest 

 where the body is widest, and thickest at the tapering posterior extremity. It is not 

 unlikely that this is in part caused by differences in the state of conti'action, but this 

 alone could not account for the very striking divergence in the ])roportionate depths of 

 the respective muscle-slieatlis [cf. PL 7. figs. 6, 8, 9, 10). 



Microscopic sections of the liody-wall serve to show tliat, in general histological 

 structure, it is almost identical with that of Echiurtts Pallasii as described by Greeff and 

 Spengel. The ejndermis is coA(ned by a well-defined cuticle, which is typical of the 

 whole group; in microsco2)ic preparations this appears as a sharp dark line running 

 along the outer border of the section, pierced by the skin-glands; and Ijeneath tliis 

 clearly marked band, and abutting on tlie epidermal cells, a vacuolated layer presents 

 itself, similar to that figured by Jameson * in Thalassema neptuni (PL 7. figs. 6, 7, 

 8, 9, 10). 



The epidermis is composed of a single layer of columnar cells, but it is difficult to 

 distinguish them as such, for they are much distorted by the pressure of the numerous 

 unicellular glands and clear cells ; the epidermal cells give the appearance of a clear 

 matrix in Avhich these darkly staining bodies S(;em to be embedded. The nuclei of the 

 epidermal and gland-cells lie at their inner ends, and, as they stain almost as strikingly 

 as tlie glandular secretion, they form a dark border in section (PL 7. figs. 0, 7, 8, 9, 10). 



Occasionally the unicellular glands assume giant proportions, become filled willi 

 secretion, and might well be mistaken for compound glands (PL 7. fig. G, (j.u.). In the 

 depressions between the papillae there are neither gland- nor " trigger-cells '' ; these 

 latter are seen scattered among the glandular tissue, being most noticcjible on the 

 proboscis (PI. 7. fig. 11). 



* "Contributions to the Anitomy and Histologj- of Thalassema nepiani, Gaertner," /ool. Jahrb. lid. xii. 1899. 



12* 



