94 Memoirs of the Indian Museum. [Vol. I, 



The cerebral ganglia give off on either side three large nerves, — the optic nerve and 

 nerves to the first and second antennse. The last two nerves leave the cranium 

 through two gaps in the cuticle, which are separated by a stout skeletal bar. The 

 gap for the first or antennular nerve, which lies above and slightly to the inner side of 

 the other, is nearly circular and measures about 2 mm. in diameter; it is surrounded by 

 an elevated ridge of skeletal material which projects into the cavity of the head. This 

 ridge is scarcely less than 2 mm. in height and nearly as much in thickness ; it is 

 continuous with the bar which separates the two gaps The ridges which surround 

 the antennular gaps on either side are united across the middle line by a short ridge 

 of equal thickness and height. This transverse ridge lying as it does, a. conspicuous 

 feature, directly between the antermular gaps, must be considered as the first cephalic 

 sternite. It is firmly based upon the internal side of the frontal lamina. 



M. Bouvier speaks of the frontal lamina as the '' segment antennulaire," while 

 the clypeus he refers to as the ' segment antennaire." This view receives support 

 from the fact that the transverse ridge, which from its position must be regarded as 

 the first sternite, is firmly fused with the frontal lamina. On the other hand, atten- 

 tion must be drawn to a deep groove separating the frontal lamina, and with it the 

 clypeus, from the rest of the head. This groove, which is shown in fig. 3, commences 

 above as the posterior limit of the frontal lamina, passes down in front of both 

 antennular and antennary gaps, to end below between the antennary tubercle and 

 a second smaller tubercle {a.t., t.). The ending of this groove between the two tu- 

 bercles is shown in Bouvier' s fig. 9 (pi. iv) ; the antennary tubercle is indicated 

 by the letter a, the other to the inner side being unlettered. 



The antennary gap, which is much larger than the antennular, measures about 

 5 mm. in diameter and is bounded below by a strong skeletal bar, continued externally 

 into a ridge forming the anterior border of the mandibular gap. 



The Structure of the Eyes. 



In the position and size of its eyes, Bathynomus is remarkable among Isopods. 

 According to Bouvier there are nearly three thousand corneal facets in each eye. 



The internal structure of the eye was studied by means of sections, both 

 vertical and tangential to the surface (pi. xii). The cuticular cornea, which com- 

 prises more than half of the total thickness of the eye, is made up of about thirty 

 closely applied layers. These layers together show in vertical section a uniform 

 wavy appearance ; the centre of depression of each wave lying over the centre of 

 each perceptive element or ommatidium. The outermost layer of the cornea differs 

 from the others ; it is denser, takes the stain with greater avidity, and is continued 

 as an uninterrupted sheet over the whole external surface of the eye. 



Between the cornea and the perceptive part of the eye is a well-developed 

 homogeneous layer, which in its intimate structure much resembles the cornea, but is 

 separated from that organ by a narrow cellular layer, — the corneagen. 



The perceptive part of the eye consists of a number of ellipsoidal bodies com- 

 posed of translucent yellow material; these are the vitrellse or lenses, each of 



