334 



ARACHNOIDEA AND PALAZ0S7RACA. 



ostia. Hsemocyanin 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 

 ring around the nerve-collar, which gives off vessels to the limbs, and 

 is continued backwards around the nerve-cord. From capillaries the 

 blood is gathered into a ventral venous sinus, whence it passes to the 

 respiratory organs, and thence to the pericardium and heart. 



The respiratory organs or gill-books are borne by the last five 

 appendages. Each looks like a much-plaited gill, or like a book with 

 over a hundred hollow leaves. The leaf-like folds are externally washed 

 by the water, and within them the blood flows. The leaves of the 



gill-books are compared to the leaves 

 of the lung-books of scorpions. If 

 this homology is correct, the gill-books 

 are evaginations, the lung-books in- 

 vaginations, of the skin. 



The reproductive system.— 

 The males are smaller than the 

 females. The testes are very diffuse, 

 the two vasa deferentia open on the 

 internal surface of the operculum, and 

 the spermatozoa, which are vibratile, 

 are shed into the water. The ovaiies 

 form two much-branched but con- 

 nected sacs ; the oviducts are separ- 

 ate, and enlarge before they open 

 beneath the operculum. 



Spawning occurs in the spring and 

 summer months. The ova and sper- 

 matozoa are deposited in hollows near 

 high-water mark. Some of the early 

 stages of development, still impei- 

 ectly known, present considerable resemblance to corresponding stages 

 in the scorpion. In the larvte, both cephalothorax and abdomen show 

 signs of segmentation, but this disappears. The spine is represented 

 only by a very short plate, and the larva presents a striking superficial 

 resemblance to a Trilobite. 



It seems likely that Limulus is linked to the extinct Eurypterids by 

 some fossil forms known as Hemiaspidfe, e.g. Hemiaspis, Belinurns. 



Fig. 144. — Young Limulus. - 

 After Walcott. 



Order 2. Eurypterina ( = Merostomata), e.g. Eurypterus. 



Gigantic extinct forms found from Ordovician to Carboniferous strata. 

 The body is divided into head, thorax, and abdomen. The head is 

 small and unsegmented. The thorax is composed of six distinct seg- 

 ments, the abdomen of six with a terminal telson, which was sometimes 

 a pointed spine, sometimes paddle-shaped. There is, however, some 

 doubt as to the exact nomenclature of the regions. On the head are 



