. Fig. 3. C. borealis antipoda. Right antenna from inside. A, female; B, endopod of male. , clasping organ; en, endopod; ex, exopod; , natatory setae; sh, shaft. (Fig. 3 B, ) 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


. Fig. 3. C. borealis antipoda. Right antenna from inside. A, female; B, endopod of male. , clasping organ; en, endopod; ex, exopod; , natatory setae; sh, shaft. (Fig. 3 B, ) 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


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Keywords: ., bookcentury1900, booksubjectocean, booksubjectscientificexpediti