INTKODUCTION. XI 



sac the cardiac division of the stomach. This sac has a system 

 of calcareous levers in its walls connected with teeth projecting 

 into the interior, and worked by appropriate muscles, so as to 

 form an efficient and powerful apparatus for the mastication of 

 food. Opening from the cardiac division of the stomach behind 

 is another sac : the pyloric division of the stomach ; the walls of 

 this division are raised into folds, covered with hairs so as to 

 form a kind of strainer to prevent any but finely comminuted 

 particles of food from passing further. The strainer opens into 

 the narrow thin-walled intestine, which extends in a straight 

 course to the anal aperture. The liver consists of two large, 

 equal, lateral halves, one lying on each side of the stomach, the 

 two bile-ducts opening into the pyloric division. 



The nervous system consists of a cerebral or cephalic ganglion, 

 situated in the front part of the head below the root of the 

 rostrum, and giving off nerves to the eyes, antennae, antennules, 

 and the parts about the head, and of a chain of nerve ganglia 

 extending throughout the length of the body. Of these ganglia 

 six are situated in the thorax, lying on the lower or ventral 

 aspect between the bases of the legs, and six lie in a correspond- 

 ing position in the abdomen. The first thoracic ganglion is 

 larger than the rest, and is connected by two nerves or com- 

 missures, one passing on each side of the oesophagus, with the 

 cerebral ganglia, and by other two with the second thoracic 

 ganglia ; the other thoracic ganglia are similiarly connected by 

 double commissures, but the commissures between the abdominal 

 ganglia are single and undivided. 



The organs of sense consist of the eyes, certain cilia on the 

 under surface of the flagella of the antennules supposed to be 

 olfactory in function, and the auditory sacs situated in the basal 

 joint of the antennules. 



The reproductive organs consist of the sexual glands {ovary 

 and testis), situated above and behind the pyloric division of 

 stomach and behind the heart, and the ducts (oviducts and vasa 

 dcferentia) by which their products are conveyed to the 

 exterior. The oviduct opens on the coxopodite of the ante- 

 penultimate thoracic appendage ; the vas diferens on the coxo- 

 podite of the last pair of appendages. 



