220 DISCOVERY REPORTS 



Nebaliopsis would be comparable with that of the feeding mechanism of the fairy shrimp Chirocephalus. 

 Now this type of filtratory mechanism is not nearly so powerful as that of Nebolia. A floating particle, 

 once it has been sucked in between the limbs of Nebalia, cannot, as far as I can see, escape unless it is 

 forcibly ejected. On the other hand, a particle, on being sucked into the mid-ventral space of Chiro- 

 cephalus, as often as not is blown out again, and it would be the same with Nebaliopsis. 



However, this is all on the supposition that the trunk limbs of Nebaliopsis are still efficient filters. 

 But I pointed out (1931, p. 21 1) that this cannot be so because, among other things, their lateral parts 

 do not form a valvular system as they do, in fact, in all other Nebaliacea. In Nebalia and Paranebalia 

 it is the epipodites together with the exopodites, and in Nebaliella, where the epipodites are absent, the 

 exopodites alone, which project backwards and completely and accurately span the gaps between 

 successive limbs, thus preventing any lateral entry of water into the inter-limb spaces during the 

 suction phase. In Nebaliopsis, on the contrary, there are wide lateral gaps both proximal and distal to 

 the epipodite through which water could pass unhindered. Linder (loc. cit. p. 30) denies this and says 

 that the gaps are simply due to the distension of the limbs. He implies, although he does not say so in 

 as many words, that the main axis of the limb has elongated under pressure, while the epipodite has 

 remained the same size, and hence a gap has occurred between the epipodite and the body wall. The 

 fallacy of this is obvious. There is no reason why the epipodite should not enlarge along with the rest of 

 the limb. In fact from the anatomy of the limb it is fairly certain that if this hypothetical swelling did 

 take place it would be the epipodite which would enlarge most and not the main axis of the limb, for 

 the latter is skeletally relatively rigid, while the epipodite is extremely delicate. 



One further argument : if the lateral gaps which are so clearly seen in the upper photograph of my 

 plate xxxii (193 1) are due to artefacts, how is it that the lateral space between the first and second trunk 

 limbs is so accurately covered by the epipodite of the first trunk limb (loc. cit. p. 206, fig. 3)? Is this 

 also due to distension? Surely not — it is simply a manifestation of suction between the first and second 

 trunk limbs which is absent, or practically so, between the other limbs. 



Having explained away the lateral gaps between the trunk limbs, Linder still has to deal with the 

 distal gaps that occur between the tips of the limbs. These gaps result from the absence of the pos- 

 teriorly curved endopodites. The typical trunk limbs ' are unsegmented, the exopodites being repre- 

 sented by a slight protuberance. . . .The endopodite must be considered as the tip of the limb distal to 

 this exopodite lobe' (Cannon, 193 1, p. 207). They are, as Linder correctly states (loc. cit. p. 10), turgor 

 limbs, that is, limbs that depend for their rigidity on an internal blood pressure. Moreover, ' the distal 

 two-thirds of each limb is devoid of musculature ' (Cannon, loc. cit. p. 210), so that the limb cannot be 

 bent by internal muscles — it can only be moved as a whole by the muscles at its base after being made 

 rigid by being pumped full of blood. 



Now Linder explains the closure of the gaps between the tips of the limbs during the 'Abduktions- 

 phase ' (suction phase) as due to a bending over. of one limb on to the limb in front, as a result of suction 

 between the limbs (loc. cit. p. 30). This would imply that directly any two successive limbs commence 

 their suction stroke, the suction between them becomes so great that it causes the hind limb to bend in 

 the middle so that its tip comes down on the limb in front and closes the distal gap. But what causes 

 this original powerful suction? Unless there are efficient valves, the suction between the limbs will be 

 negligible. The gap is there to start with and hence the small suction caused by the inter-limb space 

 enlarging will simply draw in water through the gap which will at once neutralise the suction. Linder 

 says that similar conditions obtain in the Anostraca (loc. cit. p. 30) and he sees no reason why the same 

 thing should not happen in Nebaliopsis. But things are quite different in the Anostraca. There the limb 

 is jointed and the distal endopodite ffap is provided with a complete set of muscles by which it can be 

 pulled down and held, if necessary, on to the next limb. 



