fischeri. Such minute pores or canals may have given passage to sense organs (sensory setae or bristles) or to tegumentary glands.
Serrations and spines. The edges of the integumental plates are frequently furnished with sharp serrations, like those of the abdomen of Limulus. These serrations are especially frequent on the anterior edge of the palette of the swimming leg, on the epimera of the postabdominal segments and on the lateral edges of the tail spine. In Eurypterus the distal edges of the segments of the swimming legs are also distinctly serrate. Frequently an alternating series of larger and smaller serrae is found. A good example of this is the telson of Pterygotus buffaloensis.
The spines on the legs and those on the manducatory edges of the coxal segments have originated from fine hairs, which grew first into strong bristles and finally into spines. These spines become powerful spurs in some genera, as Eusarcus and Stylonurus. In Ctenopterus, the subgenus of Stylonurus, they are sometimes broadened and may have assisted in swimming. Like the hairs, the spines are inserted and fixed by ringlike swellings of the integument at their bases and frequently broken out, leaving distinct scars.
Cephalothorax. The body of the Eurypterida consists of 18 segments and is functionally divided into two sharply distinct regions, the cephalothorax or prosoma, consisting of six fused segments and the abdomen consisting of 12 segments. The cephalothorax with its appendages is specialized for locomotion and prehension, while the abdomen is essentially a vegetative sac. The six segments of the cephalothorax find their expression in the six pairs of appendages. The cephalothorax consists of the carapace or head shield and the organs of the underside.
The carapace is of small size, one fifth of the body (exclusive of the telson) or less in length. This small size is a feature that gives the eurypterids a greatly different aspect from Limulus with its immense semicircular carapace, but it well corresponds to the relative dimensions in the scorpions. In Limulus it is expanded so much both laterally and frontally
