TR1LOB1TA. 361 



ring around the nerve-collar, which gives oft" 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 imaginations, 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-water mark. Some of 

 the early stages of development, still imperfectly known, present con- 

 siderable resemblance to corresponding stages in the scorpion. In the 

 lame, 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 Hemiaspid;?e, e.g. Heuiiaspis, Betimims. 



Order 2. EURYPTERIXA ( Merostomata), e.g. Eiirypterus. 



Large 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 \\s sometimes 

 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 compound 

 eyes, and two median ocelli. On the ventral surface of the thorax 

 there are five pairs of gills covered by flat plates, of which the most 

 anterior pair are very large, and form the so-called operculum (cf. 

 Limuhis], The surface of the body was covered with scales. Some 

 of the Eurypterids reached a length of 6 ft. 



This order is sometimes placed near the Crustacea, but the general 

 opinion is that they are linked ihroughLiwii/its to Arachnoids. 



Order 3. TRILOBITA. Trilobites, e.g. Calymene, Pkacops, 



Asaphus. 



Extinct forms chiefly found in Cambrian and Ordovician strata, but 

 extending up to the Carboniferous. The body as found is divisible into 

 three parts the unsegmented head shield, often prolonged backwards 



