2l6 



DISCOVERY REPORTS 



Such a form, I suggest, developed the maxinary-first trunk Hmb filter mechanism, 

 at first, to aid its more posterior trunk limb filter, just as I have suggested in the 

 evolution of the primitive Malacostracan the maxillary filter developed to assist the 

 trunk limb feeding mechanism (1928, p. 820). Then, when this became sufficiently 

 advanced, it opened up the carapace — maybe to allow a greater inflow of water on to 

 the maxillary region. At this stage water would enter the mid-ventral space, not only 

 anteriorly, but also ventrally. The maxillary filter now became the chief feeding 

 mechanism and, with further evolution, became greatly enlarged. As it developed so 

 the carapace widened out and water came to be sucked in from all directions. The trunk 

 limb filter was then almost abandoned, the trunk limbs swinging forwards to act as a 

 subsidiary mechanism supplying water to the primary maxillary-first trunk limb filter. 



On these arguments Nebaliopsis shows an interesting parallel to Nebalia. I pointed 

 out (1927, p. 367) that the ancestor of Nebalia was a filter feeder and that, in its evolu- 

 tion "the primitive filtering mechanism disappeared, and with the re-establishment of 

 a filtering method of feeding the original mechanism did not return, but an entirely 

 new method was estabhshed " (1927, p. 368). Now this filter mechanism of the ancestral 

 Nebaliacean, lost in Nebalia, was a maxillary filter (Cannon and Manton, 1929, text- 

 fig, i) and, in the evolution oi Nebaliopsis, a maxillary filter has been re-established, but 

 it is an entirely new apparatus — a combination of maxilla and first trunk limb, which is 

 otherwise unknown in the Malacostraca. 



Nebaliella extrema, J. Thiele 



The genus Nebaliella was established by Thiele in 1904 to include two specimens 

 collected by the ' Valdivia ' in the neighbourhood of Kerguelen, and a single incomplete 

 specimen collected about 1875 by the 'Gazelle' from the same locality. He described 

 them under the name Nebaliella antarctica. He suggested also that a specimen in the 

 Copenhagen Museum, which was collected in Akaroa Harbour, New Zealand, is a 

 young individual of this species. In 1908 he described another species, A'', extrema, 

 from fifteen specimens collected by the 'Gauss' off the coast of Kaiser Wilhelm H 

 Land. Among the Discovery material there is a single complete and undamaged speci- 

 men of this latter species collected in the Palmer Archipelago. 



The following list gives details of the localities in which Nebaliella has been found : 



Table H. 



