ANATOMY AND HISTOLOGY OF PLEUROCH ATA MOSELEYI. 485 
the hypoderm, which is represented diagrammatically, is a network of fibrous 
tissue, within the meshes of which lie the muscular fibres cut transversely, 
either singly or in groups of two or more. The strands of connective tissue 
separating the muscles appear to be elastic tissue; they are stouter in the 
lower part of the transverse coat than in the upper; towards the surface they 
become finer, but here and there stouter bands form largish compartments, which 
are again divided up by the finer bands between which lie the muscles. A 
more deeply stained continuous membrane separates the muscular from the 
hypodermic layer; beneath the fibrous bands of the circular coat are perfectly 
continuous with those of the longitudinal coat. 
The longitudinal is essentially similar to the circular coat, but the fibrous 
bands are far slighter ; in fact, unless the section is rather stretched, it is very 
easy to miss them altogether, but quite impossible in the case of the outer 
circular coat. The advantages of this elastic network to the animal must be 
enormous, giving its skin so great a flexibility, and that not at the expense of 
its muscles. It is rather difficult to compare the thickness of the two coats, as 
the presence of the elastic tissue renders them so easy of compression or of 
extension, that in the case of a given section one cannot say with certainty 
whether it really represents the skin of the animal in its normal condition or 
not. On the whole, it would appear that the two coats are about equal in 
thickness, except in the anterior part of the body where the longitudinal coat is 
considerably thicker (see Plate X XVI. fig. 15, which is taken from a young 
specimen, but would do perfectly well for the adult in this respect), and has 
a much more strongly developed fibrous meshwork (Plate X XVI. fig. 13). 
PERRIER* figures and describes a somewhat similar arrangement in Pontodrilus 
marionis, but apparently confined to the inner longitudinal coat ; the meshes 
also enclose a far greater number of fibres. 
In his researches into the common earthworm, CLAPAREDE? figures and 
describes strands of fibrous tissue separating the bundles of muscular fibres in 
the inner longitudinal coat, giving to the muscles the bipinnate arrangement so 
characteristic of the earthworm. This is very evident on examining sections 
of the integument of the earthworm; but, as PERRIER remarks, it is not 
at all generally found among the Oligochwta. Comparing, however, sections 
of the integument of Lumbricus with that of Pleurocheta, and finally with 
the figures given by Perrier of the same structure in Urochwta,{ it would 
seem that the arrangement in Luwmbricus is only an extreme modification 
of what is found in Pleurocheta, and that there exist in this latter arrange- 
* Perrier, Joc cit., pl. xvi. fig. 27. 
+ “ Histologische Untersuchungen ueber den Regenwiirm,” Zeitschrift fiir Wissensch. Zoologie, 1869. 
t Perrier, Archives de Zoologie Experimentale, vol. iii., 1874. 
