THORACOSTRACA OR PODOPHTHALMATA. 



5 6l 



thoracic limbs are eight on each side, and are provided each with a?i 

 exopodite and endopodite (fig. 421), the exopodites being natato?y in 

 function. A cephalothoracic shield is present, and there is usually 

 only a single pair of maxillipedes. The gills are attached to the 

 thoracic legs, or, exceptionally, to the abdominal feet. The ova are 

 ca?-ried beneath the thorax of the female, usually in a marsupial pouch 

 formed by leaf like plates produced from the bases of the legs (fig. 

 421, ;;/). The telson often possesses minitte terminal appendages or 

 spines. 



The Schizopods are distinguished from the Decapods by the 

 larger number of the thoracic limbs, and by the fact that these 

 appendages have well-developed exopodites, as well as by the fact 

 that the gills are not carried in branchial chambers formed by a 

 downward prolongation of the sides of the carapace. In Mysis and 

 its allies true branchiae are wanting. With the single exception of 

 the Mysis relicta of the great lakes of Sweden and North America, 

 all the Schizopods are inhabitants of the sea, 

 extending their range to considerable depths. 



No undoubted fossil forms of the Schizopoda 

 are as yet known, but a brief consideration may 

 be given here to the singular Palaeozoic genera 

 Palceocaris and Gampsonyx, which some author- 

 ities regard as aberrant members of the Amphi- 

 poda, but which are considered by Packard as 

 being probably ancestral types of the Schizopods. 

 The genus Palceocaris (fig. 422) includes peculiar 

 elongated Crustaceans from the Coal-measures 

 of North America and Britain, in which there is 

 a short cephalothoracic shield, but the hinder 

 thoracic segments are free. There are two 

 pairs of antennae, of nearly equal length, and 

 the limbs do not appear to be bifid. The 

 telson has a pair of " stylets " on each side, 

 and the condition of the eyes has not been 

 clearly ascertained. The genus Acanthotelson, 

 also from the Coal-measures, appears to be re- 

 lated to Palceocaris, as, possibly, are the genera 

 Palazoi'chestia (Carboniferous) and Nectotelson 

 (Permian), but the true relationships of these 

 forms are very uncertain. Another Carbon- 

 iferous genus which may perhaps find a place here is Gampsonyx, 

 including small Crustaceans which have a general resemblance to 

 the Amphipods in appearance, but in which the limbs seem to 

 have been of the Schizopodous type. 



Order III. Stomatopoda. — This order includes the recent 



Fig. 422. — Pala-ocaris 

 typus, slightly enlarged. 

 From the Coal - measures 

 of North America. (After 

 Meek and Worthen.) 



VOL. I. 



2 N 



