298 ZOOLOGY. 



ages are of uniform shape like legs, not like mandibles or 

 maxillae, and are adapted for walking ; the feet are pro- 

 vided with sharp teeth on the basal joint for retaining the 

 food. The mouth is situated between the second pair; the 

 first pair of legs are smaller than the others. All end in 

 two simple claws, except the sixth pair, which are armed 

 with several spatulate appendages serving to prop the crea- 

 ture as it burrows into the mud. The males differ from the 

 females in the hand and opposing thumb of the second pair 

 of feet. These cephalo-thoracic appendages are quite as dif- 

 ferent from those of most Crustacea as those of the mites 

 and spiders, which have a pair of mandibles and maxillae, 

 the latter provided with a palpus. Appended to the ab- 

 domen are six pairs of broad swimming feet, all except the 

 first pair of which bear on the under side two sets of about 

 one hundred respiratory leaves or plates, into which the blood 

 is sent from the heart, passing around the outer edge and 

 returning around the inner edge. This mode of respiration 

 is like that of the Isopods. 



The alimentary canal consists of an oesophagus, which 

 rises directly over the mouth, a stomach lined with rows of 

 large chitinous teeth, with a large conical, stopper-like valve 

 projecting into the posterior end of the body ; the intestine 

 is straight, ending in the base of the abdominal spine. The 

 liver is very voluminous, ramifying throughout the cephalo- 

 thorax. The nervous system is quite unlike that of the 

 Crustacea ; the brain is situated on the floor of the body in 

 the same plane as the rest of the system, and sends a pair of 

 nerves to the compound eyes, a single nerve supplying the 

 ocelli.* The feet are all supplied with nerves from a thick 

 ring surrounding the oesophagus. The nerves to the six 

 pairs of abdominal legs are sent off from the ventral cord. 



* The nervous system of Limulus is quite unlike that of the Scorpion, 

 where the brain is situated in the upper part of the head and supplies 

 the maxillae with nerves, and is situated directly over the infraceso 

 phageal ganglion ; and, besides, there is no O3sophageal ring as in 

 Limulus, only the two commissures connecting the brain with the 

 infrao3sophageal ganglion as usual in the Crustacea and Arachnida in 

 general. 



