I. '08. 61 



which is soluble in undiluted spirit, but fairly permanent in 

 formalin. Specimens preserved for over a year in a mixture of 

 75 per cent, formalin (5 f)er cent.) and 25 per cent, spirit have 

 lost all trace of the bright scarlet colour which is such a con- 

 spicuous feature of this and other deep-water Crustacea when 

 living, but the photophores stand out very clearly as deep 

 blue spots and streaks. In a paper in a previous report 

 (1906 (1)) will be found a description of the luminous organs 

 as they appear in a specimen of 43 mm. ; although the number 

 piesent is considerably greater in the mature animal, it seems 

 unnecessary to give here a detailed account of them ; reference 

 to the table on p. 64 and to the figures will clearly show the 

 position and numbers of those present in specimens of dif- 

 ferent sizes. Thus the larval form possesses only twelve 

 photophores (six pairs), while no less than one hundred and 

 forty-seven are to be found in the largest specimens. 



The photophore on the exopod of the first maxillipede (fig. 

 10} is the chief addition here made to the very full list given 

 by Coutiere (1906) ; it is present in specimens of 33 mm. in 

 length and upwards.^ PI. VI, fig. 1, shows an adult A. 

 debilis in lateral view with its photophores ; in large specimens 

 it is impossible from this aspect to see any of the unpaired 

 photophores, and those behind the coxa of the uropods and on 

 the inner faces of the endopods of the pleopods and exopods of 

 the pereiopods are also invisible. An attempt has been made 

 to show, by means of fine lines round the luminous spots, 

 those areas on the carapace and abdomen which are devoid of 

 pigment. These have already been mentioned by the author 

 (1907) ; they have the appearance of small windows in the pre- 

 vailing red colouring of the animal, and their purpose is doubt- 

 less to enable the light to pass out from the photophore with- 

 out being obstructed by pigmentation. 



Size. — The largest specimen examined measures 78 mm. in 

 length. 



Hansen (1908) records a specimen of this species under the 

 name of Acanth.eph.yra gracilis, which is cited in my paper of 

 1906 (1) as a synonym of A. debilis. The discrepancies which 

 this author notes between his specimen and my figure are 

 doubtless due to the fact that at that time I had only very 

 young examples at my disposal. Hansen also states that he 

 could find no vestige of photophores, but the great difficulty 

 of observing these organs in specimens which have been pre- 

 served for any length of time in alcohol probably accounts for 

 their apparent absence. Once the blue pigment, which 

 characterises them in life, has disappeared, it is only by the 

 closest scrutiny that their existence is noticed, for a definite 

 lens has only been demonstrated in the case of a single series, 

 viz., those on the protopodites of the pleopods. 



^ 1 1 have not been able to discover the photophore which Coutiere men- 

 tions on the exopodite of the second pair of maxillipedes. Maxillipede 



II. in his list is perhaps a' misprint for maxillipede I, 



