REPORT ON THE CRUSTACEA MACRURA. 365 



done to show that a long, strong tooth, somewhat similar in character to the rostrum, 

 projects from the anterior surface of the epistoma, and that the mandibles exist as sharp- 

 pointed organs, but the presence of a synaphipod could not be determined. Only one 

 pair of gnathopoda could be seen, which I took to be the second; it consists of a long 

 cylindrical basal joint supporting two branches, the permanent ramus being four-jointed; 

 each joint is furnished at its distal angle with a long ciliated hair, and the terminal joint 

 has four; the outer or deciduous branch consists of one long and one minute joint, 

 the former furnished with a long ciliated hair on the inner and one on the outer distal 

 angle, and the latter with four at the extremity. 



One feature of interest in the development of the species from which this specimen 

 comes, is that all or at least five out of the seven somites that belong to the pereion are 

 developed as perfect somites posterior to the carapace. This I also found to be the case 

 in Claus's figure of the Protozoea of Peneeus, and in his figure of Ericthina, but it is not 

 the case in Dana's figure of the typical specimens of Ericthina, which is believed by 

 Professor Brooks to be the Zoea form of the genus Leucifer. 



The next form to be considered, and which Dr. v. Willemoes Suhm says comes from 

 the latest Elaphocaris, is that which he has described as being in the Amphion-st&ge. It 

 is desirable in tracing the fife-history under the present circumstances, when one form 

 is said to succeed and arise from another, to take into consideration the relative sizes of 

 the specimens, for after allowing for the variations that may occur from various con- 

 ditions and circumstances, the difference of size is a great factor when we consider how 

 gradual are the changes in the progressive history of the stages that are known. The 

 largest Elaphocaris is about 1"7 mm., and Suhm says the Amphion-stage is 3'5 mm., or 

 more than double the length. 



The specimens in the Challenger collection that belong to the A mphion-stage of Suhm 

 in the progressive development of Sergestes, are similar to those that have been described 

 by Glaus under the name of Acanthosoma. Of these there are several varieties, as if they 

 were the young of more or less closely allied species, but there are two forms in which 

 the characters are so distinct, that I think there cannot be any doubt they are stages of 

 two different but closely allied genera. These may be arranged for convenience under the 

 names of Acanthosoma brevitelsonis and Acanthosoma longitelsonis, leaving it to future 

 research to determine the parent of either form. None of our specimens correspond in 

 minute details with that figured by Claus. That given by him in his Crustaceen 

 System (pi. v. fig. 5), of which he has represented the rhipidura only, corresponds with 

 that of Acanthosoma longitelsonis (PI. LXIV. fig. 3), whereas his figure 6, on the same 

 plate, corresponds with that of Acanthosoma brevitelsonis. Professor W. K. Brooks 1 

 says: — "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." 



1 Phil. Trans., pt. i. p. 102, 1882. 



