SYSTEMATIC REPORT 13S 



(4) Antennal scale small, styliform and not leaf-like. 



(5) The seven posterior pairs of thoracic appendages almost equal in length with three -segmented 

 propodus and well developed dactylus. 



(6) Exopods of first thoracic appendages lacking. 



(7) Female pleopods rudimentary, those of male all well developed and normal. 



(8) Telson small, egg-shaped, apex truncate. 



(9) Endopods of uropods shorter than exopods. Statocyst well developed. 



Ortmann in his description of the species recorded that the uropods were twice as long as the telson. 



Stebbing (1905, p. 114) recorded the species from 190 fm. about 47 miles off Cape Town and 

 Zimmer (1914, p. 397) recorded specimens taken by the German South-Polar Expedition, 1901-3. 

 He discussed the close affinity of this genus to Echinomysis Illig and pointed out that the essential 

 differences between them lay in the form of the antennal scale and of the exopods of the first thoracic 

 appendages. In Echinomysis both these organs, though small, are well developed and normal. 

 Ortmann had stated that there was no trace of an exopod on the first thoracic appendage in Caesaro- 

 mysis, but Zimmer discovered that a quite readily recognizable vestige was present as well as a large 

 well developed epipod (Fig. 31C). 



The exopod and endopod of the thoracic appendages in the Mysidacea arise from the third segment 

 of the sympod, i.e. the basis, and the epipod, when present, arises from the second segment of the 

 sympod, the coxa. In Caesaromysis the basis of the first thoracic appendage has become fused with 

 the body-wall along the greater part of its anterior margin, while the coxa has become so closely fused 

 with it and with the body-wall as to be indistinguishable. A considerable lengthening of the body has 

 taken place in the region of the first and second thoracic somites and, as a result, the attached basis of 

 the first thoracic appendage has also become lengthened. That portion bearing the endopod has re- 

 mained closely associated with the mouth parts, while the portion bearing the exopod and the coxa 

 with its epipod has become widely separated from it and remains attached to the body-wall just in 

 front of the second thoracic appendage. If one places the animal on its side and gently lifts the cara- 

 pace one can see the whole peculiar structure of the appendage clearly. The tiny finger-like vestige 

 of the exopod projects downward from the body in its normal position, while the large basin-shaped 

 epipod extends backward under the carapace covering the bases of the succeeding thoracic append- 

 ages. A well-marked ridge, which is really the median portion of the basis of the first thoracic append- 

 age, can be clearly seen running forward to the proximal end of the endopod immediately behind the 

 mouth. 



In Echinomysis the exopod of the first thoracic appendage is normal and is borne in the normal 

 position beside the endopod. 



Considerable sexual differences occur in Caesaromysis and, whereas in the females and in immature 

 specimens the antennal scale is reduced to a simple papilla-like vestige, in adult males it is well 

 developed and similar to the scale in Echinomysis. 



Zimmer and Illig showed that considerable sexual differences exist in the proportions of the uropods 

 and in their length, relative to that of the telson. Their investigations were restricted by the lack of 

 fully adult males and as a result they could only indicate the tendency to sexual differentiation. The 

 beautifully preserved material of the Discovery collections contains adults of both sexes and I have 

 been able to figure those parts of the male which are of special interest. I am now able to draw up 

 an amended definition of the genus in the light of our present knowledge : 



(1) Cephalothorax broad and flattened dorso-ventrally ; abdomen short and relatively small. 



(2) Rostrum produced anteriorly into a very long powerful spine, lateral margins armed with long 

 spines. 



