94 COMPARATIVE ANATOMY 



inflation is known as the periophthalmic sac, or water-sac, and 

 its anterior end is prolonged into a fine canal which runs 

 forward and opens in the middle line by a small pore a short 

 distance in front of the eyes. Lying between the posterior 

 lobes of the periophthalmic sac is a conspicuous white oval 

 patch of a glandular nature. This is called the neck gland ; 

 it opens to the exterior by a very small pore placed near its 

 front end. Neither the neck gland nor the pore of the 

 periophthalmic sac must be mistaken for the median unpaired 

 eye, often called the nauplius eye, which is a minute black 

 spot situated between the anterior horns of the paired eyes. 



In front of the eyes the cephalic shield slopes rather 

 abruptly downwards, and on turning the specimen over, so as 

 to examine the ventral surface, it will be seen that the anterior 

 margin of the shield is thickened to form a broad flat" plate 

 lying in front of the mouth, as shown in fig. 21. The mouth 

 itself is concealed by a large pentagonal chitinous plate, the 

 upper lip or labrum, the anterior end of which is hinged to 

 the thickened rim of the cephalic shield. The anus is ter- 

 minal, lying at the end, and somewhat on the ventral side of 

 the furcal piece. 



On cutting away the free part of the cephalic shield close to 

 its attachment at the nuchal groove the elongate vermiform 

 body is exposed. The body proper that is to say, the region 

 behind the mouth, is to be regarded as made up of thirty-nine 

 annuli or segments ; the first five of these are fused with and 

 indistinguishable from the head, and their existence can only 

 be determined by the appendages which they bear and the 

 nerve ganglia corresponding to them. But behind the attach- 

 ment of the cephalic shield thirty-three distinct annuli can be 

 counted in addition to the terminal or caudal segment. Each 

 annulus is indicated externally by a hoop of chitin, the posterior 

 edge of which overlaps the annulus next behind it. These 

 hoops are simply chitinised thickenings of the integument, and 

 the hinder edge of one is joined to the anterior edge of the 

 next behind it by a soft fold of the integument which is 

 doubled back under the overlap and so forms a flexible joint. 

 All joints in the external armour of Arthropoda are formed in 

 this manner, and may be compared with the ancient suits of 

 plate armour, consisting of overlapping pieces of steel sewn on 

 a leather jacket. The chitinous hoops are thin, transparent, 



