igo8.] R. E. Li^OYD : Bafhynomus giganteus. 87 



below. This examination, which gave a negative result, must have disclosed any 

 small duct passing between the gland and the pharynx, if existent. The opening of 

 these glands is probably among folds of the cuticle on either side of the mouth. The 

 general appearance of a section of one of these organs is shown in the photograph (pi. xii, 

 fig. 6). The lumen of the tubules composing the organ is lined by a delicate chitinous 

 intima. Salivary glands have been previously described in Isopods (7, 18). 



Homologies of the alimentary system. 



Although, at first sight, the gizzard of Bathynomus appears considerably different 

 from those of the smaller Isopods, consideration shows that this difference is 

 more apparent than real. A conspicuous feature of the gizzard of such genera as 

 Asellus and Armadillidium is a pair of large ampullœ, which project into the cavity 

 of the organ in its anterior part. The ampullae of the gizzard of Asellus seem to be 

 homologous with the anterior ampullae in Bathynomî4S, for in both cases the principal 

 muscle of the gizzard is attached to the concave side of these structures. The pos- 

 terior or large ampullae of Bathynomus are not represented in the smaller genera. 

 The principal pair of ossicles in the gizzard of Asellus lie in the fore part of the floor 

 of that structure and together form a V-shaped figure. These seem to be represented 

 in Bathynomus by the transverse ossicles, which lie, one on either side, beneath the 

 muscular ampullae. The transition from the arrangement of these structures in 

 Asellus to that in Bathynomus, by the widening out of the limbs of the V until they 

 are both in one transverse line, is easy to imagine. 



In the case of the mid gut and hind gut, the homologies between Bathynomus 

 and its smaller allies are less clear. As shown in text-fig. i, which is copied from 

 Murlin (8), the mid gut of Oniscus is relatively of great length, comprising by far the 

 greater part of the whole alimentary canal. This condition is also found in Asellus 

 and other small Isopods so far as they are known. In Bathynomus the relative size 

 of the divisions of the alimentary canal contrasts with this, for in this genus the 

 hind gut is greater than the combined length of the first two divisions. As shown in 

 the text-figure, the extensive mid gut of Oniscus is not of the same structure throughout 

 its whole length. There is a typhlosole in its anterior part. Posterior to this the mid 

 gut shows a curious arrangement of its cells, described by Murlin in the following 

 words : — 



"Another striking feature of the epithelium (of the mid gut) is the rectangular 

 arrangement of the cells in longitudinal and transverse rows. One exception occurs 

 at the posterior end of the t3^phlosole, at which point the longitudinal rows con- 

 verge, so as to form, as Schönichen says, ' parallel parabolas, making a picture in 

 optical section not unlike a longitudinal section through a vegetative point ' " 

 (see text-fig. i). Again, in speaking of the muscular coat of the mid gut, he says, 

 "The muscular coat has been fully described by Ide and Schönichen. It consists of 

 two layers, an outer longitudinal and an inner circular. Over the anterior portion 

 of the mid gut {i.e., as far back as the typhlosole extends) the outer is imposed upon 

 the inner ; posterior to this both layers thin out so that the fibres are quite widely 



