PHOTOPHORES OF DECAPOD CRUSTACEA 



365 



pleopod organs is interesting as bringing S. debilis into agreement with the condition 

 found in S. affints. It is also noteworthy that similar cells are found in the organs on the 

 fifth thoracic limb of S. debilis. 



{c) The photophores on the fifth thoracic limb 

 The photophores on the fifth thoracic limb of 5. debilis present a striking contrast to 

 those found in the similar position in S. affinis. There, it will be remembered, the 

 photophores consist of segmented spherical masses of a white granular substance (p. 356), 

 and differ essentially from those behind the base of this limb, but in S. debilis the 

 photophores of the limb have a very similar structure to that at its base. 



For the purpose of description the organ borne by the propodus is selected (Fig. 29). 

 It consists of photogenic cells of the now familiar type, showing a cup-shaped nucleus 

 (Fig. 29, n.phot.c), a large area of the cell devoid of cytoplasm (Fig. 29, phot.c.cl.a.), 



100 



V 



phot.c.b.c. 

 n.chit.ep. 



Fig. 29. Longitudinal section through the fifth thoracic Hmb of Systellaspis debilis, to show the structure of 

 the photophore on the propodus. The haemocoele is indicated by mechanical stippling. Mallory's triple 

 stain, chit, chitin ; cut. cuticle ; f.s. fibrous sheath ; m.l. muscles of the limb ; n.chit.ep. nucleus of chitogenous 

 epithelium; n.phot.c. nucleus of photogenic cell; phot.c.b.c. basal cap of photogenic cell; phot.c.cl.a. clear 



area of photogenic cell. 



8-2 



