NEBALIELLA 217 



Thiele does not figure or give measurements of A^. extrema but from his account it 

 is to be presumed that it did not differ markedly in general shape and size from N. 

 aiitarctica. He gives measurements of the latter species but, unfortunately, they do not 

 tally with his drawing. According to his figures the carapace should appear larger pro- 

 portionately to the rest of the body than in his illustration. If this correction is made the 

 general shape and proportion of the body agree with the Discovery specimen of A'', extrema. 



The chief dimensions of the Discovery specimen are : 



Carapace length ... 

 Carapace depth ... 

 Length of rostrum 

 Length of flange of rostrum 

 Width of flange of rostrum 



Length of eye ... ... 



Length of caudal furca, excluding terminal spines ... 2-1 ,, 



Total length from tip of rostrum to tip of caudal furca 11-5 ,, 



The three main characteristics of the genus Nebaliella are the peculiar form of the 

 rostrum and eyes, the fact that the third and fourth joints of the antenna are separate 

 and armed with stout spines, and the absence of epipodites on the trunk limbs. Of 

 these, the latter distinguishes Nebaliella from all other Nebaliacea, but, from a functional 

 standpoint, it is the least important characteristic. 



The morphology of the trunk limbs has been described in detail by Thiele (1904, 

 p. 7; 1905, p. 64). Apart from the absence of epipodites they are closely similar to the 

 trunk \\n\hsoiNebalia. I have studied their arrangement in the Discovery specimen 

 and find it identical with that of Nebalia. As will be seen from Fig. 6, the specimen is an 

 adult female carrying embryos. These were held by the tips of the endopodites curving 

 backwards and inwards to form a complete basket under the trunk and a floor for the 

 filter chamber between the trunk limbs. The walls of the filter chamber are constituted 

 as in Nebalia, each trunk limb carrying four rows of setae along its inner margin. The 

 first and third rows are long and form the actual filter wall. The second consists of a few 

 long feathered setae near the base of the limb and the fourth row, of short setae on the 

 hinder face of the inner margin, projecting inwards and backwards through the first 

 row setae of the limb behind. I could not settle whether the first and third rows of 

 successive limbs interlocked as in Nebalia as, in removing the embryos in order to 

 study this setation, the setae were naturally displaced. The eighth pair of trunk limbs 

 are close together in the middle line and form the hinder wall to the filter chamber just 

 as in Nebalia. 



On cutting away the carapace on one side, I found that, although the epipodites were 

 absent, the exopodites were, proportionately to Nebalia, much larger, each exopodite 

 completely covering the lateral space between its limb and the limb behind. That is, 

 the exopodite of Nebaliella has taken the place of the combined exopodite plus epipodite 

 of Nebalia which I showed (1927, p. 358) acted as valves allowing the passage of water 

 from the filter chamber between the limbs to the exterior at the hind end of the carapace. 



In Nebaliella then, the trunk limbs are arranged in the same way as in Nebalia, their 



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