296 ARTHROPODS. 



Order ii. Eurypterina (= Merostomata), e.g., Eurypterus, 

 Pterygotus. 



Extinct forms found from Ujiper Silurian to Carboniferous strata. 

 The cephalothorax (?) bore five to six pairs of appendages, two lateral 

 compound eyes, and two median ocelli. The abdomen (?) consisted of 

 twelve segments without appendages. At the end of the body was a 

 spine-like or rudder-like telson. Some of them measured many feet in 

 length. 



Some incline to keep the Eurypterids near Crustaceans, but the 

 favourite opinion seems to be that which links them through Limitlus to 

 Arachnoids. 



Order 12. Trilobita. Trilobites, e.^., Calymene, Phacops, Asaphus, 

 Agnostus. 



Extinct forms chiefly found in Cambrian and Silurian strata, but 

 extending to the Carboniferous. There was a cephalothorax with four 

 pairs of limbs around the mouth, and sometimes bearing dorsal compound 

 eyes. The abdomen was divided into an anterior segmented and a 

 posterior unsegmented region, both with limbs. 



The body was externally divided by a median longitudinal ridge into 

 three longitudinal portions. Trilobites are often found rolled up in a way 

 that reminds one of some wood-lice. So abundant are they in some 

 rocks, that even their development has been studied with some success. 



The limbs appear to be liker those of Crustaceans than of Arachnoids, 

 but the present balance of opinion seems to be in favour of placing them 

 nearer Arachnoids. It is possible that they are offshoots from a stock 

 ancestral to both. 



Appendix to Arachnoidea : — 

 PaNTOPODA or PYCN"OGONID.?i. 



These are marine Arthropods, sometimes called sea-spiders. Their 

 affinities are uncertain, but perhaps they may be ranked between 

 Crustaceans and Arachnoids. Many climb about seaweeds and hydroids 

 near the shore, but some live at great depths. The body consists of an 

 anterior proboscis, a cephalothoracic region with three fused and three 

 free segments, and an unsegmented rudimentary abdomen. In most 

 there are seven pairs of apjjendages, into five of which outgrowths 

 of the mid-gut extend. The sexes are separate, and the males usually 

 carry the eggs attached to the third pair of appendages. The larvce 

 are at first unsegmented, with three pairs of appendages. 



Examples : — Pycnogonum, Nymphon, Ammoihea. 



