48 THE LIFE OF CRUSTACEA 



from outside, but is formed in position by secretion 

 from the walls of the sac. 



Most of the Mysidacea have no special organs of 

 respiration, that function being discharged (as in 

 many of the smaller Crustacea) by the general 

 surface of the body, and especially by the thin 

 carapace ; but certain deep-sea Mysidacea (Fig. 17) 

 have tufted gills attached at the base of the thoracic 



FIG. 17 Gnathophausia willemoesii, ONE OF THE DEEP-SEA MYSI- 

 DACEA. HALF NATURAL SIZE. (From Lankester's "Treatise on 

 Zoology," after Sars.) 



gr, A groove dividing the last abdominal somite 



legs. In all cases the maxilliped has a plate-like 

 epipodite, which lies under the side-fold of the cara- 

 pace, and no doubt assists respiration, causing by its 

 movements a current of water to flow under the 

 carapace. 



The members of the second order of the Pera- 

 carida, the CUMACEA (Fig. 18), are small marine 

 Crustacea in which the anterior part of the body 

 is generally stout, while the abdomen is slender and 

 very mobile. The short carapace does not cover 



