MOEPHOLOGY OF CYCLOPS. 5 



The carapace, from the region of the antenntc backAvard, and all the thoracic segments 

 but tlic sixth, have each distinct pleura prolonged behind downwards and backwards into 

 auricles, most marked and acute in the fifth (fourth free) thoracic segment. In front the 

 carapace or tergum of the cephalon and first thoracic segment is continued downwards 

 into a blunt narrow ventral rostrum, separated from an oblong epistoma by a narrow 

 vertical cleft (notadnate as often stated). Just above the rostrum on each side, in front 

 of and above the sockets for the antennules, are the nearly circular, smooth, flattened 

 corneal facets (PI. II. fig. 7), between which, on the anterior end of the carapace, are a 

 few symmetrically placed circumvallate setae, probably representing the frontal organ. 



The sternal region (PL II. fig. 3) is much more diversified. In the cephalon we may 

 distinguish the median region from the lateral areas occupied by the sockets for the limbs. 



The antennulary sockets are almost lateral to the rostrum. . Between the antennules 

 is a narrow ridge, the epistoma (PL III. figs. 1, 4, 6), acute and V-shaped in section in 

 front, becoming broad and bilobed behind, extending more and more downwards, and 

 passing into the broad labrum. 



The mouth (PL III. figs. 4, 6) is a squarish opening between the two mandibles, 

 bounded in front by the labrum, behind by a transverse prneoral bar, continuous with 

 the thickened socket of the mandible. From the sides of this bar spring the two 

 paragnathpB, or lobes of the "metastoma" of Huxley, which underlie the shafts of the 

 mandibles, and form practically a floor to the mouth. All these parts will be described 

 more fully in connection with the digestive apparatus. For the rest of the cephalon this 

 median part of the sternal region is broad and flattened. 



Laterally two sinuous " arthrodial ridges " (PL I. fig. 14) are formed on each side, 

 strongly chitinized. On each side, the inner and outer ridge have the concavities of their 

 sinuosities facing to form sockets for the appendages, while transverse processes bound 

 these sockets in front and behind ; but the only socket where they meet to form a complete 

 bar is that for the antenna {at") small and rounded. The others are incomplete in front 

 and behind, and that of the two maxillipeds is incompletely separated by a process from 

 the bar which joins the inner and outer arthrodial ridges behind them. The inner ridge 

 is, indeed, incomplete at the anterior side of the socket of the outer maxilliped, probably 

 owing to the inward and upward prolongation of the posterior inner angle into a 

 hollow, flattened, boot-shaped, postmaxillary apodeme {ap.'pm.), with the " foot ' 

 projecting forwards and inwards and the " heel " backwards and outwards. 



This postmaxillary apodeme* gives attachment to the great flexor muscles of the trunk 

 in all the Copepoda I have examined. 



The free entosternite (PL I. fig. 15) is a small plate in the middle line of the maxillary 

 region, flattened and concave from side to side on its upper surface ; its lower surface is pro- 

 duced to an edge, right and left of the middle ; its anterior edge is concave, and the pos- 

 terior convex, so that it forms a segment of a ring; to the outer third of its posterior edge 

 is inserted a strong muscle connecting it with the postmaxillary apodeme— the retractors 

 of the entosternite ; between these it is produced into a median plate with its hinder 



* Lankester considers apodemata -with a hollow opening to the free surface of the bofly as an Araclmoid character. 

 " Limulas an Arachnid," in Q. J. M. S., 1S82. 



