102 MR, W. K. BROOKS ON LUCIFER: 



The telson is deeply cleft, and its halves diverge from each other like a swallow's 

 tail feathers, so that the posterior ends of the rudimentary swimmerets are visible 

 between them in a dorsal view, as shown in figs. 77, 78, and 79. The spines on the 

 telson are similar in arrangement to those of Lucifer, but much longer. 



A comparison of the Lucifer Zoea (fig. 44), the Acetes Zoea (fig. 79), and the 

 Sergestes Zoea (CLAUS'S 'Crustacean-System,' taf. vi., fig. 1) at the same stage of develop- 

 ment, brings out the extremely interesting fact that the Acetes larva stands between 

 the very simple Zoea of Lucifer and the remarkably complicated Elaphocaris larva of 

 Sergestes in nearly every feature in which, the two differ. In Lucifer the eyes are 

 sessile ; in Acetes they have short stalks ; and in Sergestes the stalks are very long. 



In Lucifer the spines over the eyes are absent ; in Acetes they are present and 

 simple ; and in Sergestes they are very long and compound. 



In Lucifer the postero-lateral spines are parallel to the long axis of the body ; in 

 Acetes they are oblique, so that they project a little beyond the outline of the body ; 

 and in Sergestes they are at right angles to the long axis, and compound. 



The carapace, including the rostrum, makes about one-third of the total length of the 

 body of the Lucifer Zoea ; about one-half of that of the Acetes Zoea ; and more than 

 two-thirds of the total length of the Sergestes Zoea. The abdominal somites of the 

 Acetes Zoea are shorter and wider than those of the Lucifer Zoea, and this change is 

 carried still further in the Sergestes Zoea. In the Lucifer Zoea the sixth abdominal 

 somite is the only one which has ventral spines, and these point backwards. All the 

 abdominal somites of Acetes have spines, and they point backwards and a little 

 outwards, while in Sergestes they all point directly outwards. 



The telson is slightly notched in Lucifer ; deeply forked in Acetes ; and in Sergestes 

 the prongs of the fork diverge so much as to form a right angle. 



These facts are extremely interesting, as they seem to show that the Elaphocaris is a 

 larva essentially like that of Lucifer, which has passed through a remarkable process 

 of secondary modification, resulting in the acceleration of the development of the 

 eyes, and the production of a forked telson, and a very 'spiny body. The larva of 

 Acetes has been modified in the same direction but to a much less degree. It may be 

 asked why we are to assume that the Lucifer Zoea is the primitive form, and the 

 Elaphocaris larva the secondary modification rather than the reverse ; but a little 

 thought will show that the distinctive features of the Elaphocaris stand in direct 

 relation to the environment, as weapons of defence, sense organs, or locomotor 

 apparatus, while the distinctive marks of the Lucifer Zoea are features of general or 

 typical resemblance to the corresponding larva of Euphausia and Penceus. 



I did not succeed in finding the Proiozoea from which the Zoea shown in fig. 79 is 

 derived, but I think it extremely probable that future research will show that an 

 unknown larva which has been figured by DOHRN and GLAUS is the Protozoea of Acetes, 

 or else of a new closely-related genus of the Sergestidae. 



In his " Untersuchungen iiber Ban und Entwickelung cler Anthropoden" (Zeit. f. 



