"Crustacea with numerous, free, thoracico-abdominal segments, the first and second (?) of which bear one or more broad lamellar appendages upon their ventral surface, the remaining segments being devoid of appendages; anterior rings united into a carapace, bearing a pair of larval eyes (ocelli) near the centre, and a pair of large marginal or sub-central eyes: the mouth furnished with a broad post-oral plate, or metastoma, and five pairs of movable appendages, the posterior of which form great swimming-feet: the telson, or terminal segment, extremely variable in form; the in-tegment characteristically sculptured" (Henry Woodward).
The Eurypterida are all extinct, and are entirely confined to the Palaeozoic period. Many of them attained to a comparatively gigantic size; Pterygotus Anglicus (fig. 151) being supposed to have reached a length of probably six feet. In their characters they present many larval features; resembling the larvae of the Deca-poda especially in the fact that all the free somites of the abdomen (except the two anterior ones) were totally devoid of appendages.
Fig. 152. - Larva of Limulus on hatching, greatly enlarged. (After Dohrn.)