SYSTEMATIC REPORT 33 



fact, it is amply afforded by the rich range of the species in the Discovery collection. The specimens 

 described by these earlier workers varied considerably in size, but if their descriptions and figures 

 are compared with animals of about the same length in the present material they agree closely in every 

 particular. 



Growth changes. In G. ingens the following growth changes take place: (1) in the relative length of 

 the rostrum and the various spines which arm the carapace, (2) in the number of teeth on the outer 

 margin of the antennal scale, and (3) in the shape and size of the epimeral plates of the sixth abdominal 

 somite. (1) In young specimens of less than 32 mm., the rostrum is relatively very long, nearly 50% of 

 the length of the animal. The branchiostegal spines are also very long and widely extended laterally ; 

 the posterior median spine extends backwards over the abdomen to the posterior margin of the sixth 

 abdominal somite; and the postero-lateral spines are even longer, extending backwards in an arc 

 to the level of the apex of the telson. As growth proceeds, the rostrum and all the spines on the 

 carapace become relatively smaller, until in specimens of over 130 mm. the rostrum is only 16% of the 

 length, 1 the branchiostegal spines become shorter and are directed backwards and less sideways and the 

 posterior dorsal spine is very short and may be almost obsolete. The postero-lateral spines extend 

 backwards only to the level of the second abdominal somite. (2) In young specimens the outer margin 

 of the antennal scale bears at most two small teeth in addition to the terminal spine. The number of 

 these teeth increases as the animal grows until there are from 6 to 8. (3) Interesting changes in the 

 shape and relative size of the epimeral plates of the last abdominal somite have been described by 

 Ortmann (1906). In some species of Gnathophausia {ingens among them) these epimera bend under 

 the body and meet in the mid-ventral line. At first they are simply contiguous, but soon become 

 united to form a rectangular plate covering the anterior half of the ventral surface of the somite. 

 The posterior margin of this plate is somewhat concave, the posterior angles not very acutely pointed 

 and the line of fusion is marked only by a faint ridge. Gradually the posterior margin becomes more 

 emarginate and the distal end of the whole plate narrows, while the two angles lengthen to form two 

 acutely-pointed lappets. As these lengthen, their inner margins become straighter and the fissure 

 between them is reduced to a narrow slit which is slightly dilated at its proximal end. 



When the animal has reached a length of about 65 mm. a small outgrowth begins to develop on the 

 inner side of each lappet near the distal end and, so narrow is the fissure at this stage, that the two 

 lappets actually overlap distally. This secondary tooth-like outgrowth continues to grow, but does 

 not equal the lappet in length and in really large animals the lappets appear asymmetrically bifid with 

 the larger fork on the outer side. 



After a length of over 100 mm. has been reached, there is no further change in the form of the 

 epimeral plate. 



Some confusion has arisen in the past because in the Challenger female specimen of G. ingens 

 Sars (1885 a) figured the fissure of the epimeral plate as fairly wide with straight inner margins and 

 no dilatation at its proximal end. The lappets are shown as bifid, with the larger and longer point on 

 the inner side. Hansen (1927, p. 16), when recording a gigantic male of 210 mm. captured by 

 ' Talisman ' south of the Azores, noted that its epimeral plate differed from that of the Challenger 

 specimen. The fissure was narrow, so that the inner margins of the lappets were touching except 

 proximally, where there was a definite dilatation. The lappets were bifid with the larger prong on the 

 outer side. A re-examination of the Challenger specimen revealed that Sars's figure was correct for the 

 fissure, but incorrect as regards the lappets which were actually similar in every way to those of 

 Hansen's males. In the largest males in the Discovery collection, the whole form of the epimeral 

 plate is similar to Hansen's figure of the Talisman specimen, but in the largest Discovery females the 



1 Measured from the apex of the rostrum to the apex of the telson. 



