3 o 4 DISCOVERY REPORTS 



antero-laterally with the body just behind the antennules and extends straight forward. The multi- 

 articulate antennal exopod with its long natatory setae is similar in both sexes. It articulates somewhat 

 to the outside of the apex of the shaft, the articulation corresponding exactly in position with the base 

 of the antennal notch of the carapace (Fig. 2). It can be extended through this notch or withdrawn 

 into the carapace and then lies beneath the body of the animal, with its natatory setae embraced by 

 the palps of the mouth-parts and the bases of the first trunk-limbs. 



The broad, flattened basal article of the endopod (Fig. 3) articulates with the shaft on its inner 

 surface near the apex. It bears two short setae which differ in structure in different species. The 

 second article bears two setae of moderate, but unequal length, of which at least the apices are thin- 

 walled. Besides these, in the male there are three very short setae, but not more than one of them is 

 present in the female; in the female of Conchoeia borealis antipoda they are all absent. The end-article 

 bears three 'tube-setae' similar to those of the antennule. In the male there is a clasping organ 



Fig. 3. C. borealis antipoda. Right antenna from inside. A, female; B, endopod of male. 

 c.o, clasping organ; en, endopod; ex, exopod; n.s, natatory setae; sh, shaft. 



(Fig. 3 B, c.o) which Skogsberg (1920) considered to be a modification of the end-article itself. This 

 is a hook-like structure which differs somewhat in form on the two sides of the animal, and also 

 differs in different species. Normally the longer setae of the ramus extend vertically downwards, just 

 within the anterior gape of the carapace ; the ramus can, however, be moved antero-posteriorly. 



The main adaptation of the antenna is for swimming. The exopod of the appendage can be extended 

 through the antennal notch and moved freely, even when the valves of the carapace are tightly closed. 

 The form of this notch and hollowing of the carapace below and behind it allow free backward move- 

 ment of the exopod. The exact match in position of the articulation between the antennal exopod and 

 shaft with the notch leads to the notch serving as a rowlock for the backward stroke of the oar-like 

 exopod and its natatory setae. The articulation is bicondylar, with the one condyle ventral and the 

 other dorsal but slightly displaced outward and forward. The main part of the musculature enclosed 

 in the antennal shaft is the flexor musculature of the exopod. Skogsberg (1920) described the swimming 

 action of the antennae and pointed out that it resulted in a forward propulsive stroke. In fact, the 

 bicondylar articulation leads to the exopod swinging back in an arc with a slight downward movement. 

 This downward movement gives a slight uplift to the front of the body exactly like that produced by 

 our arm action in the breast stroke. Uplift in the water will also be produced by the boat-shape of 



