EURVPTE RINA—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 larve, 
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- 
aspidee, e.g. Hemiaspis, Bélinurus. 
Order 2. EURYPTERINA (=Mero- 
stomata or Gigantostraca), eg. 
Lurypterus 
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 F1G. 200.—Young L2mulus.— 
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. Ldveulus), 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 Zzmzlus to Arachnoids. 
Order 3. TRiLopiTA. 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. 
