28 A BIOLOGY OF CRUSTACEA 



currents produced by its thoracic limbs for respiratory purposes. 

 The outer branches (exopods) of the thoracic limbs are held out 

 sideways from the body and each one is rotated in such a manner 

 as to produce a vortex with its centre along the centre about which 

 the limb rotates. Water swirls up the vortex and bathes the gills 

 which lie at the bases of the legs, those of one side meeting those 

 of the other in the midline. 



In some other mysids, such as Hemimysis, the pleopods are 

 reduced and the thoracic limbs have taken over the function of 

 locomotion, and, further, they produce an auxiliary feeding current. 

 Again the exopods produce a vortex and water swirls up from the 

 tip of the limb towards the body (fig. 20B). Now Hemimysis does 

 not have gills, so that a clear space, or groove, is left along the 

 midline, between the two rows of limbs. Water from the two sides 

 of the body meets in the midline and passes forwards, partly due 

 to the forward inclination of the opening at the bases of the legs 

 and partly due to suction resulting from movements of the second 

 maxilla. The food particles carried along by these currents are 

 trapped on the filter setae of the second maxillae and so enter the 

 basic feeding mechanism already described. 



Gnathophansia and Hemimysis are not restricted to feeding on 

 small particles; they can eat large particles when the need arises. 

 The inner branches (endopods) of the thoracic limbs hold large 

 particles against the mandibles, which bite into the food material 

 and pass it upwards into the oesophagus. 



The rotation of the thoracic exopods also propels Hemimysis 

 through the water. This is done by virtue of the arrangement of the 

 setae at the tips of the exopod, which are arranged so that they 

 spread out and offer great resistance to the water as the exopod 

 moves backwards, but close together and offer minimal resistance 

 as the exopod moves forwards. A special respiratory current is 

 produced by a small flap at the base of the first leg. This flap, or 

 epipodite, projects into the space between the body and the cara- 

 pace, and by its movements it draws water in under the dorsal 

 hind edge of the carapace, and pushes it out at the sides of the 

 two pairs of maxillae. Hemimysis does not have special gills, but 

 the carapace is richly supplied with blood spaces, which bring the 

 blood into close contact with the respiratory current. As the respira- 

 tory current leaves the sides of the maxillae it gets swept upwards 

 behind the eye (fig. 20A) and then backwards with the main swim- 

 ming current. 



