340 



ARACHNOIDEA AND PAL/EOSTRACA. 



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 append- 

 ages. Each looks like a much plaited gill, or like a book with over a 

 hundred hollow leaves. Their leaf-like folds are externally washed by 

 the water, within them the blood flows. The leaves of the gill books 

 are compared to the leaves of the lung Ijooks of scorpions. If this 

 homology is correct the gill books are evaginations, the lung books 

 invaginations, 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 

 ovaries form two much branched 

 but connected sacs ; the oviducts 

 are separate, and enlarge before 

 they open beneath the operculum. 



Spawning occurs in the spring 

 and summer months. The ova 

 and spermatozoa are deposited in 

 hollows near high w^ater mark. 

 Some of the early stages of de- 

 velopment, still imperfectly known, 

 present considerable resemblance 

 to corresponding stages in the scor- 

 pion. In the larvae, both cephalo- 

 thorax and abdomen show signs of 

 segmentation, but these disappear. 

 The spine is represented only by 

 a very short plate, and the larva 

 presents a striking superficial re- 

 semblance to a Trilobite. 



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

 some fossil forms known as Hemiaspids, e.g., Hcmiaspis, Beliimnis. 



Fig. ioS. — Young Limulus. 

 (After Walcoit.) 



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 

 segments, the abdomen of six with a terminal telson, which was some- 

 times a pointed spine, sometimes paddle shaped. There is, however, 

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

 are borne six pairs of appendages of varying shape, two lateral com- 

 pound eyes, and two median ocelli. On the \entral surface of the 

 thorax, there are five pairs of gills covered by flat plates, of wdiich the 



