106 ESSENTIALS OF ZOOLOGY 



degenerates, forming the striated muscles. The cavities of 

 the gonads and the end sac of the green gland alone survive 

 as coelomic cavities. Their ducts are mesodermal in origin, 

 and are therefore coelomiducts. 



Nervous System. The nervous system consists of a brain, 

 the cerebral or supra-oesophageal ganglion, connected with a 

 ventral chain of ganglia. The brain is placed on the front wall 

 of the head between the eyes. It gives off paired nerves to the 

 eyes, to the antennules and to the antennae, and is connected 

 with the ventral chain by two long oesophageal commissures, 

 which pass backwards on each side of the oesophagus to unite 

 in a large sub- oesophageal ganglion. The latter is made up of 

 the fused ganglia of the segments represented by the mandibles, 

 maxillae, and the first two pairs of maxillipeds. The ganglion 

 representing the third maxilliped segment immediately 

 succeeds the sub- oesophageal ganglion, and then follow the 

 five ganglia of the pereiopod segments, and the six abdominal 

 ganglia. The ventral ganglia receive and transmit impulses 

 affecting their own segments, but they are all under the co- 

 ordinating control of the brain. 



In addition to the ganglia and the nerves associated with 

 them, there is a visceral or so-called sympathetic system. 

 This consists of two nerves passing backwards from the brain, 

 internal to the oesophageal commissures, to the oesophagus, 

 and supplying the stomach, liver, and perhaps also the heart. 



The large reniform compound eyes are situated at the ends 

 of the movable eye-stalks. The latter are sometimes con- 

 sidered a first pair of appendages, and it is remarkable, at all 

 events, that when the whole eye-stalk is artificially removed 

 an antenna grows in its place. Each eye consists of a large 

 number of similar elements, or ommatidia, radiating outwards 

 from the optic ganglion at the end of the optic nerve. The 

 nerves proceeding from the ganglion pierce a basement mem- 

 brane and enter the basal sensory elements of the ommatidia 

 the spindle-shaped, striated rhabdome, made up of four cells 

 enclosed by the retinula cells. The rays of light are trans- 

 mitted in each case by the crystalline cone of four cells, which 

 end in a point internally, the corneal ectoderm and the flat 

 square facet of the transparent cornea. The cornea is shed at 



