PAL^EOSTRACA 



429 



of the exopodite and endopodite structure characteristic of 

 Crustaceans. 



Each " gill-book " 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 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 Hemiaspidae, 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 eves, and two 



median ocelh. On the ventral surface of the thorax there are 

 five pairs of gills covered bv flat plates, of which the most 

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

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

 Some of the Eurvpterids 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. 



Fig. 



241. — Young Limulus. — 

 After Walcott. 



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- 



