SPECIFIC CHARACTERS 475 



The early post-larval Grimothea stage of M. gregaria (Plate IV, fig. 5) differs from 

 the adult in addition to the above-mentioned characters, in size, being about 7-12 mm. 

 in length of carapace as against the 20-38 mm. of the adult. The exoskeleton is much 

 softer and less calcified. The spines and imbricating scales of the carapace, of the 

 abdominal segments and of the limbs are more feebly developed than in the adult. The 

 breadth of the carapace between the antero-lateral spines is relatively greater. The 

 chelae and endopodites of the external maxillipeds are longer in proportion and there is 

 a greater development of setae upon them (Plate IV, fig. 5). The Rev. T. R. R. Stebbing 

 (1919, p. 330) records and figures an early larval stage of M '. gregaria 2 mm. in length. 

 I am unable to find any other description or figure of the larval stages of either M. gregaria 

 or M. subragosa, though Young (1925, p. 319) records from MSS. at the Otago Fish 

 Hatchery that Anderton hatched out larvae of M. gregaria on September 28, 1908 and 

 October 4, 1914. 



The stage in the life history when the Grimothea of M. gregaria assumes the adult 

 characters and takes to the bottom appears to vary. The largest Grimothea in the 

 Discovery collections measures 12 mm. in length of carapace (St. 51), while the smallest 

 M. gregaria with adult characters measures 13 mm. (Plate IV, fig. 6) in length of 

 carapace (St. 51). On the other hand, the specimens taken from swarms at the surface 

 off the Patagonian coast vary in carapace length from 18 mm. (S.S. ' Ernesto Tornquist ', 

 27. iii. 29) to 33 mm. (S.S. 'Ernesto Tornquist', 5-iv. 29) (Plate IV, fig. 3). These 

 appear to be adult, but all have a number of Grimothea characters, so that they are almost 

 intermediate between the Grimothea stage and the bottom-living adult. Allowing for 

 the fact that the specimens are preserved in formalin, whilst the remainder of the col- 

 lection, with the exception of the Grimothea specimens from St. WS 100, are preserved 

 in spirit, they nevertheless appear to have been less heavily calcified than specimens 

 taken on the bottom. In addition they have a smaller development of spines, particularly 

 those on the antero-lateral corners of the carapace, the chelae are longer and more 

 slender and have a greater development of setae, as also do the external maxillipeds, 

 which are longer in proportion to the total body length (cf. Plate IV, figs. 2 and 3). 



This confirms in part the views of Chilton (1909, p. 612, see below, p. 479). He argues 

 that the Grimothea stage may remain pelagic for a longer or shorter time, depending on 

 the abundance of food at the surface and the availability or otherwise of a suitable 

 bottom to which the Grimothea may descend and become adult. He thinks that the 

 Grimothea may become bottom-living at an early age or, alternatively, may continue 

 pelagic and become sexually mature and breed at or near the surface. The small specimen 

 with adult characters from the bottom, and the large ones with Grimothea characters 

 from the surface, recorded above, support this view. They do not, however, uphold 

 Chilton's contention that M. subrugosa and M. gregaria are one and the same species. 

 The abundance of the large surface-living form off the coast of Patagonia is probably 

 due to a plentiful food supply, but what other biological or physical factors determine 

 whether the Grimothea shall continue pelagic or take to the bottom are unknown. Off 

 the Patagonian coast the swarms do not remain pelagic because there is no suitable 



