PHOTOPHORES OF DECAPOD CRUSTACEA 363 



(ii) Systellaspis debilis 



The structure of most of the photophores of S. debilis has already been described by 

 Kemp (1910&, p. 643, as Acanthephyra), and the following notes deal only with some 

 additions and corrections based on an examination of formalin preserved material from 

 the Dana Expedition. 



In these specimens the photophores are clearly visible as purplish red streaks and 

 spots, but all other pigmentation has been lost. Accounts of the number and distri- 

 bution of the photophores have already been given by Coutiere (1906) and Kemp 

 (1910a), and will not be added to here. 



[a) The pleopod and uropod photophores 



After examination of the pleopod photophores of Systellaspis debilis I am able to 

 endorse fully the description of them given by Kemp (igiob, p. 643) and to give 

 additional observations. 



It was suspected by Kemp that the photophores were possibly free to undergo a 

 certain amount of movement with respect to the limb, but it was stated that no trace of 

 any controlling musculature could be found. A thick (100//) Mallory-stained section 

 of a photophore shows clearly that it possesses a musculature identical with that of 

 Hoplophoriis grimaldii (p. 333 and fig. 12), consisting of a longitudinal muscle and a 

 circular muscle loop anchored on the outer side of the limb. In the photograph given by 

 Kemp (19106, pi. lii, fig. i) the longitudinal muscle appears to be visible parallel to and 

 on the outer (left) side of the photophore nerve, but no trace of the muscle loop is 

 apparent. 



A further point of interest concerns the lens, which after Mallory staining does not 

 show clearly the three layers mentioned by Kemp (19106, p. 644). The whole lens 

 stains an almost uniform light blue in thin sections, and the layers can only be dis- 

 tinguished with difficulty by their diflFering densities of striation. The outermost layer 

 is very dense and closely striated, the middle has a comparatively loose appearance and 

 coarse striation, while the inner layer, although not as dense as the outer, is very closely 

 striated. The staining reactions of the lens, however, are very capricious, for in thick 

 sections it often readily takes up acid fuchsin in preference to aniline blue. 



Whereas the photophores situated at the bases of the uropods in Systellaspis affinis 

 differ markedly from the pleopod organs in their lack of photogenic nuclei, in S. debilis 

 they more closely resemble them. They possess a feebly developed lens derived from 

 the integument, although no special muscles are seen. The nuclei of the photogenic cells 

 show the gradation in size and abrupt truncation characteristic of the pleopod photo- 

 phores. The organs closely resemble those in the same position in Hoplophoriis. The 

 discrepancy between the structure of the uropod organs of these two species of Systel- 

 laspis is very curious, and is not readily accounted for. 



