134 MORPHOLOGY OF INVERTEBRATE TYPES 



described above. The lobster possesses also a sympathetic or 

 visceral nervous system consisting of four ganglia and of nerves 

 given off by them. Two of the ganglia, forming a pair, are 

 situated at the sides of the oesophagus and are called commissural 

 ganglia. Of the other two ganglia one is called the cesophageal 

 ganglion, is median in position and single; the other is similarly 

 median and single, and is called the gastric ganglion. The 

 sympathetic nervous system supplies nerves to the alimentary 

 canal, heart, and other viscera. 



Besides the innumerable sensory hairs which are distributed 

 all over the body and its appendages, the lobster has a pair of 

 balancing organs or statocysts and a pair of compound eyes. 

 A statocyst is a little sac in the coxopodite of the antennules and 

 communicates with the outside by means of an opening. Inside 

 the sac is a horseshoe-shaped sensory ridge composed of some 

 seventy-five plume-like hairs and about three hundred short 

 seta. Scattered among the hairs and setae are numerous small 

 statolyths. These are simply grains of sand which the lobster 

 introduces into the sac through the opening. This is done dur- 

 ing the so-called fourth larval stage. The eyes are situated at 

 the end of movable eye-stalks, and belong to the type of com- 

 pound eyes, i. e., are made up of upward of fourteen thousand 

 ommatidia. Each ommatidium is composed of a corneal lens, 

 a crystal cone formed by four cells, two pigment cells surrounding 

 the cone and shutting out excessively inclined rays of light, 

 seven retinula cells and a rhabdome or rod secreted by the latter. 



Reproductive system. The sexes are separate, but the 

 male and female are externally very much alike. The position 

 of the genital openings and the structure of the first (and to 

 some extent of the second) pair of pleopods make, however, 

 the recognition of the sex simple. 



Male. The openings of the sperm ducts are on the 

 coxopodites of the fifth pair of pereiopods (i4th somite) and 

 are directed backward and outward. The sternite of the four- 

 teenth somite forms a deep V-shaped groove for the reception 



