EURYPTERINA TRILOBITA. 377 



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 often compared to the leaves of the insunk lung- 

 books of scorpions. 



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 present considerable resem- 

 blance to corresponding stages in the scorpion. In the larvae, 

 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 Hemi- 

 aspidse, e.g. Hemiaspis, Belinurus. 



Order 2. EURYPTERINA ( = Mero- 

 stomata or Gigantostraca), e.g. 

 Eurypterus 



Large extinct forms found from 

 Cambrian 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. On the head are borne six 

 pairs of appendages of varying shape, 

 two lateral compound eyes, and two 

 median ocelli. On the ventral surface FIG. 200. Young Limulus. 

 of the thorax there are five pairs of After Walcott. 



gills covered by flat plates, of which 



the most anterior pair are very large, and form the so-called operculum 

 (cf. Limulus}. The surface of the body was covered with scales. 

 Some of the Eurypterids reached a length of 6 ft. The oldest 

 Merostomes are referred by Walcott to a sub-order Limulava somewhat 

 divergent from other Eurypterids. 



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

 opinion is that they are linked through Limulus to Arachnoids. 



Order 3. TRILOBITA. Trilobites, e.g. Calymene^ Phacops, 

 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 back- 

 wards at the angles ; the flexible thorax of a varying number of 

 segments ; the unsegmented abdomen or pygidium. A median 

 longitudinal ridge, or rachis, divides the body into three longitudinal 

 portions. 



