PHOTOPHORES OF DECAPOD CRUSTACEA 315 



the lenses of the posterior thoracic photophores. At all events the account given by 

 Welsh and Chace should be accepted with caution. (I have since received information 

 from Dr M. D. Burkenroad that through the kindness of Drs Welsh and Chace he has 

 been privileged to examine their sections, which show that the supposed photophores 

 are actually "the expanded terminal portion of the vas deferens with its included 

 secretions. This portion of the male genital apparatus of S. tenuiremiis does not differ 

 particularly from the same structure in males of other species of the genus.") 



Dr Kemp made a number of observations on these luminous liver tubules on board 

 the ' Discovery ' and has allowed me to consult and quote from his notes. He remarks 

 that the species of Sergestes can be separated, according to colour, into two groups, 

 those which are uniformly red, and those which are red anteriorly with the last three 

 abdominal somites, tail fan and limbs colourless. All the specimens of the known 

 Atlantic species belonging to this latter or "half-red" group taken by the 'Discovery' 

 possess two pairs of large organs, one pair anterior and situated above the maxilla, and 

 one above the last gill. These organs may be luminous. In two cases {S. sargassi and 

 S. armatum (?)) he describes them as having their bases deep purplish black or black, 

 and their upper parts shining red. They are undoubtedly organs of Pesta. 



I have had the opportunity of examining specimens of S. coruiciihim, S. sargassi, 

 S. diapontms, and S. edwardsi, all of which possess these organs. Burkenroad (1938, 

 p. 317) states that in S. corniculum there are as many as ten distinct groups of modified 

 liver tubules : all of these I have identified and examined. In this species, in addition to 

 the usual large antero-lateral pair, a smaller antero-median organ is situated between and 

 a little behind them, and posteriorly a prominent postero-lateral pair have between 

 them a smaller postero-median organ. Situated between these two groups of organs 

 are a further two pairs, lateral in position, making up the full complement of ten 

 mentioned by Burkenroad. 



In both S. diapontiiis and S. edwardsi there are five distinct organs ; an antero-lateral 

 pair, one single antero-median, and a postero-lateral pair. S. sargassi lacks the single 

 antero-median organ, having only the antero-lateral and postero-lateral pairs. 



Since the structure of the modified liver tubules in the four species I have examined 

 appears to be identical, the following description is based on the condition found in 

 S. corniculum, of which I had most specimens. Fig. 2 indicates the position of the antero- 

 lateral pair of organs as seen in a transverse section of the animal : the antero-median 

 organ occurs some little distance behind the plane of this section. It will be seen that 

 the organs (Fig. 2, lum.l.) are borne on the ventral surfaces of two anteriorly directed 

 lateral horns of the liver (Fig. 2, dig.L), the individual tubules projecting forwards and 

 downwards, and coming into close proximity to the roof of the branchial chamber 

 anteriorly. It is true that, as Burkenroad says, they are visible to the exterior through 

 the wall of the branchial chamber, but apart from this any light they may produce 

 must be most intense in the anterior part of the branchial chamber, since they are only 

 separated from it by a delicate sheet of the integument. The lateral and postero-lateral 

 organs also must, if luminous, illuminate the posterior part of the branchial chamber to 



