XIPHOSURA. 



793 



2--V- 



of which the anterior pair (named aortic arches), arching over 

 the intestine, supply a continuous investment of sinuses to the 

 whole of the central nervous system, and some of its branches. 

 There is a very complete system of arteries and veins, and the 

 blood on its return journey to the heart enters two great longi- 

 tudinal trunks (11), from which it flows to the six abdominal 

 appendages, in the five posterior of which it is aerated. From 

 these it is returned to the pericardial sinus (A. Milne-Edwards). 



The blood is of an indigo blue colour owing to the presence of 

 haemocyanin. It contains granular amoeboid corpuscles. 



The Excretory 

 organs, long known 

 as the ' ' brick-red 

 glands," consist of 

 four lobed masses, 

 lying ventrally in 

 the cephalothorax, 

 opposite a p p e n - 

 dages II-V, and of 

 a longitudinal tract 

 uniting them. From 

 the posterior part of 

 this a convoluted 

 duct leads, accord- 

 ing to Patten, to a 

 papilla at the base 

 of the fifth pair of 

 legs. Until the dis- 

 covery of this 

 papilla by Tower, 

 the duct was sup- 

 posed to be closed 

 in the adult. In- 

 ternally the duct 

 communicates by a funnel-shaped opening with a cavity in the 

 posterior lobe of the gland, the remains of the coelom of the 

 fifth thoracic segment. In the embryo, lobes are also situated 

 in the first and sixth segments. 



The Reproductive organs have a similar disposition in the 

 two sexes. The testes and ovaries consist of systems of ramifying 



-10 



FIG. 515. Diagram of the circulatory system of Limulus. 

 1 Oesophagus ; 2 heart with eight pairs of ostia : 3 " aortic- 

 arches " ; 4 frontal artery ; 5 marginal artery : 6 colla- 

 teral artery, receiving four (paired) vessels from the 

 heart ; 7 supra-anal artery ; 8 arterial sinus investing 

 the ventral nerve cord ; 9 nerve to one of the legs, at its 

 base the arterial trunk investing the nerve is indicated ; 

 10 sub-anal artery ; 11 longitudinal trunk giving off 

 vessels to the abdominal appendages ; 12 a nerve without 

 a vascular sheath ; 13 ventral nerve cord contained in 

 arterial sheath ; 14 lateral eye (modified from Leuckart, 

 after Milne Edwards). 



