92 Memoirs of the Indian Museum. [Vot,. I, 



to its nature. The ganglion and commissures were, consequently, removed, stained 

 and mounted in toto. On examination under the microscope, it was at once apparent 

 that those parts of the commissures which lay in contact beneath the alar plates 

 contained groups of nerve cells. There were four pairs of these ganglionic groups 

 in this situation, corresponding to the four post-oral appendages of the cephalon. 

 Bach pair of these cell groups was connected across the middle line by nerve fibres, 

 and from each group an external nerve was given off (text-fig. 8, i — 4). 



The ganglion indicated by the letters s.o.g. (fig. 4) is, therefore, in all probability 

 the first thoracic ganglion. I^ike the other thoracic ganglion it gives off two pairs of 

 lateral nerves which supply, in this case, the chief flexor muscle of the trunk close to 

 its attachment to the cephalon (see fig. i). 



Consequently, the ganglion indicated by the letters g.t. 7 is the first of four 

 abdominal ganglia. The position of the ganglia in relation to their respective seg- 

 ments is by no means regular. 



Bach cerebral ganglion shows division into two lobes ; the anterior of these gives 

 off the optic nerve and from its lower surface a small nerve to the first antenna ; the 

 posterior lobe gives off a nerve to the second antenna. 



The Internai, Skei^eton of the Head. 



On account of its size, Bathynomus is a good type in which to observe the internal 

 skeleton of the head, a structure not very well known among Isopods. This part of 

 the skeleton has been previously studied in the smaller Isopods, by examining the 

 decapitated head under the microscope, after treatment with caustic potash. It was 

 first described by Schöbl, in the genus Haplophthalmus, who named it the "kiefer- 

 zungengerust " (19). Sars has figured a similar apparatus in the case of Asellus. 



In Bathynomus this apparatus consists of two pairs of wing-like plates composed 

 of rigid chitinous material, which are attached to the inner aspect of the cephalon, 

 and project into its interior. They serve for the attachment of muscles. One pair 

 is developed from the dorsal or tergal part of the cephalic shell, and the other from the 

 ventral or sternal part. These plates will, therefore, be referred to as the sternal and 

 tergal alae. The sternal alse are firmly united with, and probably derived from, a stout 

 skeletal bar, which is seen externally on the lower aspect of the head separating the 

 maxilliped from the second maxilla. (This bar can be seen in pi. iv, fig. 9, of 

 Bouvier's monograph.) Within the head the sternal alse of opposite sides meet in the 

 middle fine, and are firmly united by tough fibrous material. By their union, a plate- 

 like structure is formed, upon which the gizzard rests. 



For descriptive purposes, this chitinous plate may be divided into anterior and 

 posterior portions, which are separated by a groove; both portions consisting of right 

 and left halves united by fibrous material in the middle line. The anterior portion 

 occupies a position not far removed from the horizontal, but each half inclines slightly 

 upwards on either side. The upper surface of this part of the plate gives attachment 

 to muscles which are inserted into the anterior ampullae of the gizzard. The border 

 which limits the plate in front is widely V-shaped, and embraces the oesophagus. The 



