MEROSTOMATA. 



553 



fissure on the under side of the head, surrounded by the toothed 

 bases of the limbs, and provided behind with a large undivided 

 metastoma (fig. 416, ;;/), while in some cases a hypostome or upper 

 lip appears to be present as well. 



Behind the carapace the body consists of thirteen free segments, 

 counting the telson as one. By Claus and Hall the whole of these 



belonging to the abdomen. By other 



are 



regarded 



mrM^m^MM^^MM 





V-/ J 



^wNr-"* 



Fig. 416. — Underside of the head and the following segments of Pterygotus Anglicus, restored. 

 o, The large compound marginal eyes ; an, The first pair of appendages, chelate, and repre- 

 senting antennae; g?i, Three pairs of slender " gnathopods," with spinose bases, placed on the 

 sides of the mouth-opening (according to Fr. Schmidt, there are four pairs of these organs); 

 mx, the last pair of appendages (" maxillipedes "), developed into great swimming-feet, but 

 acting by their bases as jaws ; in, Metastoma, drawn as if it were transparent, so as to show the 

 toothed bases of the last pair of appendages below it ; op, Operculum. The "scale-marking" is 

 only represented on the segments behind the operculum. (After Henry Woodward.) 



authorities, however — as, for example, by Fr. Schmidt — the anterior 

 six of these segments are regarded as belonging to the thorax, while 

 the posterior seven are looked upon as forming the abdomen. Im- 

 mediately behind the carapace, on the inferior aspect of the body, 

 is a broad lamellar plate, which is divided into two lateral halves by 

 a narrow median process (fig. 416, op\ and which is known as the 

 " operculum." This plate, like the structure which bears the same 

 name in the King-crabs, doubtless protected the openings of the 



