SCIIIZOPODA. 473 



In their outward appearance the Scliizopoda resemble the long- 

 tailed Decapods, inasmuch as they possess an elongated and usually 

 compressed body, a large cephalo-thoracic shield covering the thoracic 

 segments more or less completely and a well-developed abdomen. 

 In the structure of their maxillipeds and thoracic legs, however, they 

 differ essentially from the Decapods and approach the more advanced 

 larvse of the prawns, which they also resemble in their simpler 

 internal organization. Further, in all the deep sea forms the cephalo- 

 thoracic shield leaves a greater number of the thoracic segments 

 free (Siriella), and in the early larval stages all the thoracic seg- 

 ments are free as in Nebalia. A larger or smaller number of these 

 free segments subsequently fuse on the dorsal side with the carapace 

 (Gnathophausia). 



Appendages. The first three pairs of thoracic appendages (the 

 honiologiies of the maxillipeds of the Decapoda} are biranious 

 ambulatory legs and resemble in structure the following thoracic 

 legs, which, by the possession of a multiarticulate setigerous exopodite, 

 are adapted both for swimming and for producing currents in the 

 water. The two anterior pairs, however, show a closer relation to 

 the oral appendages by their shorter and stouter form and by the 

 presence of processes on the basal joint (Mysis, Siriella}. The 

 principal rainus (endopodite) of the leg is always relatively slender 

 and ends with a simple Aveak claw or with a multiarticulate tarsal 

 flagellum. Rarely (Eupliausici) the two last pairs of thoracic legs are 

 entirely rudimentary, except as regards the largely developed bran- 

 chial appendages. The abdominal legs are usually small and 

 delicate in the female, but are strongly developed in the male. 

 Sometimes they are of abnormal size and form (to assist in copula- 

 tion), but only exceptionally (male of Siriella) bear gills. The 

 appendages of the sixth segment, which is usually very much 

 elongated, are always lamellar, biramous structures and form with 

 the telson a powerful caudal fin (fig. 371). The inner lamella or 

 endopodite of this pair of limbs frequently contains an auditory 

 vesicle. 



The differences between the males and females are so great that 

 formerly they were placed in distinct genera. The former possess, 

 on the anterior antennae, a comb- shaped prominence bearing a great 

 number of olfactory hairs ; and, owing to the larger size of the 



1867. G. 0. Sars, " Carcinologiske Bidrag til Norges Fauna. Mysider," 

 Christian, 1870 and 1872. R. v. Willcmoes-Suhm, " On some Atlant. Crus- 

 tacea," cf. Trans. Lin. Soc., 1875. 



