360 



PHYL UM A K THR OPODA . 



(8-12) Under the operculum lie five pairs of flat plates bearing 

 remarkable respiratory organs (" gill - books "). These 

 appendages show hints of the exopodite and endopodite 

 structure characteristic of Crustaceans. At any rate in the 

 young they serve also as swimming organs. 



As in the scorpion, there is an internal skeletal structure, or endo- 

 sternite, lying between the gullet and the nerve-ring, serving for the 

 attachment of muscles. It should be noted, however, that an analogous 

 structure occurs in Apus and some other Crustaceans. 



The nervous system. The supra-oesophageal brain gives off 

 nerves to the eyes. United to the brain are two ganglionated and 

 transversely connected commissures forming a long oval cesophageal 

 ring, giving off nerves to the limbs, and continued into a ganglionated 

 abdominal cord. Ensheathing ring, ventral cords, and some of the 



nerves, are numerous blood vessels. 

 There are two " compound" eyes 

 lying towards the sides of the 

 cephalothoracic shield, and in front 

 of these two more median simple 

 eyes. The compound eyes are cov- 

 ered by a layer of chitin continuous 

 with that of the shield, and the 

 various eye elements are so remark- 

 ably distinct from one another that 

 the eye might be called a group of 

 simple eyes. 



The food canal. Worms and 

 the like, seized by some of the 

 pincers, are partly masticated by 

 the bases of the five posterior 

 cephalothoracic legs. The mouth 

 leads into a suctorial pharynx, with 

 chitinous folds ; thence the fore-gut 

 bends upwards and forwards into 



^^^^ a crop. Separated from this by a 



valve is the mid-gut, which extends 



along the cephalothorax and abdomen, and in the former bears two 

 pairs of large yellow hepato-pancreatic outgrowths. The hind -gut is 

 short, and ends in front of the base of the spine. 



Two large reddish glands in the cephalothorax open in young 

 forms at the bases of the fifth appendages. They also open internally, 

 and may be compared with the coxal glands of spider and scorpion, 

 with the shell gland of Entomostraca, and with nephridia (?). 



The vascular system. The heart lies within a pericardium, and 

 is partially divided into eight chambers, with eight pairs of valved 

 ostia. Hrcmocyanin is present as usual as the respiratory pigment of 

 the blood, and there are oval corpuscles. From an anterior aorta, like 

 that of the scorpion, two vessels are given off which bend backward, 

 unite with lateral arteries from each chamber of the heart, and form a 

 collateral vessel on each side of the heart. These unite in a posterior 

 dorsal artery. From the anterior aorta two other branches unite in a 



FlG. 177. Young Limulus. 

 After Walcott. 



