PHOTOPHORES OF DECAPOD CRUSTACEA 361 



therefore require no detailed description. The segmented spheres are of about the same 

 size as those in the thoracic Hmb, and in the shorter streaks are arranged in a single or a 

 double row, with occasional regions where three or more rows or a confused area may 

 be seen. In general the segmentation of the spheres is not quite so clearly defined as in 

 the thoracic limb organs, and in addition, just as in that organ, the white substance 

 is also irregularly deposited in the neighbourhood of the spheres. 



That photophores of this type, differing so much from those in the corresponding 

 position in Hoplophorus, should be found in Systelhspis affinis is a surprising fact in 

 view of the similarity between the pleopod photophores of these two forms and the 

 resemblance between the carapace and pleopod photophores of Hoplophorus. Stdl 

 more surprising is the observation (p. 366) that the carapace photophores of Systellaspis 

 debilis are essentially different from those of S. affinis ; although simpler they have the 

 same basic structure as those of the three species of Hoplophorus examined. 



(/) The pigmented spots on the telson and sixth abdominal somite 

 Red-pigmented spots on the telson of Systellaspis affinis (Fig. 20, r.s.) are mentioned 

 by Dr Kemp in his notes (Caiman, 1939, p. 191) and also in a letter to me which 

 accompanied the specimen I examined. Three spots are placed in line dorsally above 

 the base of the telson, but close examination revealed the presence, confirmed by 

 sectioning, of an additional pair of spots placed dorso-laterally at the rear of the last 

 abdominal somite immediately in front of the outer spots of the telson. The pigment 

 spots continued to be visible during the preservation of the specimen in the origmal 

 alcoholic fluid of unknown composition. On transference to fresh 70 % alcohol m 

 preparation for sectioning they completely disappeared, and in their absence no trace 

 of the opaque white substance characteristic of all the photophores other than those of 

 the pleopods was visible. Transverse sections through the pigment spots show that they 

 are associated with simple but well-defined structures. 



In the neighbourhood of the pigment spots the integument and chitogenous epithe- 

 lium present a normal appearance. The chitin (Fig. 28, chit.), consisting of two layers 

 differing in density of striation, stains a light Cambridge blue with Mallory's triple 

 stain, and is overlain by the delicate cuticle (Fig. 28, cut.) which stains with acid fuchsm. 

 The nuclei of the chitogenous epithelium (Fig. 28, n.chit.ep.) are elongated perpen- 

 dicularly to the integument, and may reach 30 /^ in length. Cell limits are only slightly 

 apparent, and the cells rest on a definite basement membrane (Fig. 28, b.m.), beneath 

 which the fibres of a connective tissue sheet lie horizontally. Spindle-shaped and 

 vacuolated connective tissue nuclei (Fig. 28, n.c.t.), differing greatly from the nuclei of 

 the chitogenous epithelium, are prominent among the fibres. 



The position of each pigment spot is occupied by a dense convex mass of nuclei 

 projecting into the body space. This nuclear mass is formed in part by the nuclei of the 

 chitogenous epithelium, but mainly consists of nuclei of quite different appearance 

 (Fig. 28, n.m.). Whereas the nuclei of the chitogenous epithelium are rounded and 

 densely and uniformly packed with chromatin, those constituting the greater part of 



