A NEW SPECIES OF PENTASTOMUM. 177 



to its histology. It is lined by a single layer of nucleated epithelial cells, and 

 the remainder of the wall is made up of two indistinctly cellular layers, in 

 which I failed to distinguish any definite muscular elements (figs. 5 and 10). 



The wall thus composed is about 042 mm. in thickness near the head, but 

 it commonly increases to about double this, and is much more compactly con- 

 stituted in the more posterior portions of the body. 



Two mesenteries (fig. 5) support this portion of the alimentary canal ; these 

 are not diametrically opposite to each other, but are separated by an angle of 

 90° to 120°. They are thin lamellae (0*021 mm. thick), made up either of small 

 rounded cells, or else of fibres placed longitudinally ; on an average, two of 

 these make up the thickness of the lamina. The mesenteries commence as 

 continuous membranes about one and a half millimetres from the hinder 

 extremity of the body, although traces of them are visible in transverse sections 

 posteriorly to this. 



In addition to the stomach, they support the hook-glands (PI. XXVII. fig. 

 10), and in the anterior portion of the body the vasa defer entia come into rela- 

 tion with them, as will be more fully explained when speaking of those organs. 



There would appear to be in L. tamiaides nothing homologous to the struc- 

 ture just described, unless it is to be found in a number of distinctly muscular 

 fibres which arise from the stomach, and spreading out in the form of a fan, 

 unite with the muscular wall of the body ; * but the differences between these 

 two sets of structures are so many, that it would scarcely be justifiable to 

 regard them as homologous, save as the result of an embryological inquiry. 



4. The Rectum (PL XXVII. fig. 7) is about 075 mm. in length ; in section 

 it is compressed dorso-ventrally, its transverse diameter being 017 mm., while 

 its upper and lower surfaces are almost in contact ; it is lined with cuticle, 

 0*015 mm. in thickness, prolonged from the external surface of the body, below 

 which is a layer of cells, forming part of the chitinogenous layer. Muscular 

 elements seem to be entirely absent. 



Secretory Organs. 



Three distinct sets of organs must, in the present state of our knowledge, 

 be grouped together under this head, although they differ widely in structure 

 and probably in function also. 



They are — 



1. The Hook-Glands. 



2. The Parietal Cells. 



3. The Stigmatic Cells. 



1. The Hook-glands (Hakendrusen, Leuckart) are undoubtedly the same 



* Loc. cit., p. 59. 



