Segments of a Lobster. 



99 



FIG. 56. 



three openings, the mouth, the terminal opening 

 of the digestive canal in the middle line of the last 

 joint of the tail on the under surface, and the open- 

 ing of the egg-producing organs at the base of the 

 third pair of walking limbs. Through this last the 

 eggs are extruded, and are carried in clusters under 

 and around the bases of the hind series of feet 



In the large anterior mass of the body, sheltered 

 by the dorsal shield, there are fourteen segments 

 united, comprising the head, 

 thorax, and abdomen. The 

 head segments bear their six 

 pairs of appended limbs, the 

 first pair of which are modi- 

 fied into stalks for the eyes, 

 which are remarkable organs, 

 each consisting of a large 

 number of rods of a crystal- 

 line appearance, each placed 

 at the end of a nerve fibril or 

 thread, and surrounded 

 by a mass of pigment The numerous united fibrils 

 of the optic nerve pass in the centre of the stalk, 

 and each fibril ends in its crystal rod, the mass of 

 rods being arranged in a cluster, slightly divergent 

 so as to exhibit a rounded outer surface, over which 

 the chitinous skin extends as a fine, perfectly trans- 

 parent covering. 



The second pair of limbs are feelers called the 

 antennules or lesser antennae, consisting of three 

 basal joints, terminated by a pair of slender processes 

 each made up of many little rings ; these are followed 



Vertical section through the eye 

 of an Insect, showing the stalk 

 or optic nerve, the white radiat- 

 ing lines or secondary optic 

 nerves and the crystal cones. 



H 2 



