REPORT ON THE CIRRIPEDIA. 15 



thorax the full-grown Copepod has five abdominal segments, which are not furnished with 

 appendages. 1 



The seven-appendage-stage in the development of the Copepoda Claus rails the 

 Metanauplius-stage. According to him, the Malacostracous Crustaceans pass through the 

 same stage. In the two groups (Copepoda and Malacostraca) the two pairs of antennae and 

 the mandibles are homologous ; the maxillae and the maxillipeds of the Copepoda are re- 

 presented in the other group by two pairs of maxillae; finally, the first two pairs of rowing- 

 feet of the Copepoda correspond to the two pairs of maxillipeds of the Malacostraca. 



Let us now return to the comparison of the Copepoda with the Cirripedia. Suppose the 

 hypothesis of Claus is correct, then the mandibles develop from the third pair of larval appen- 

 dages, and the fourth pair produces the maxillae and also the second maxillae; the fifth par 

 of appendages might then, as Pagenstecher 2 supposed, change into the first parr of 

 cirri, in which case the other five pairs of cirri might be considered as homologous with 

 the five thoracic feet of the Copepoda. To accept this supposition, it would, however, 

 be necessary, Claus says, to prove that the fifth Nauplius-appendages could not possibly 

 be lost by the Cirripedia. 



If, on the contrary, the fourth pair of appendages develops into the maxillae, and the 

 fifth pair into the second maxillae, or if, as seems to be the opinion of Claus, both 

 pairs of maxillae develop from the fourth pair of appendages, the fifth pair being lost, 

 then the five thoracic feet of the Copepoda must be considered as homologous with the 

 first five pairs of cirri of the Cirripedia. Finally, Claus is inclined to suppose that 

 the knobs on the genital segment of the Copepoda, which may be shown to be a 

 rudimentary sixth pair of rowing-feet, 3 correspond to the sixth pair of cirri of the Cirri- 

 pedia. While the highly rudimentary genital knobs of the Copepoda, which ordinarily 

 have totally disappeared, and which, when present, consist merely of a single articulation, 

 are taken into consideration, the so-called caudal appendages of the Cirripedia, which in 

 Alepas, e.g., are eight-jointed, and which must be considered as a branch of a rudi- 

 mentary seventh cirrus (Gerstaecker), are totally ignored. In conclusion, Claus says, 

 that it does not matter much whether the one or the other hypothesis (viz., that of Pagen- 

 stecher or his own) proves to be true; they are of equal value for the main question, for in 

 both the body of the Cirripedia is directly compared with the body of the Copepoda, and 

 both hypotheses acknowledge the same appendages and the same segments for the body of 

 Copepoda and Cirripedia. When both hypotheses can be true, of course they may both 

 be false also. As long as neither of them is based on directly observed facts, the 



1 Clans. Lehib. d. Zool., 4th Aull., 1880, p. 544. 



- Pagenstecher, Beitrag Zur Anat., &c, Zeitschr.f. wiss. Zool., Bd. xiii., 1863. 



:i " Welcher nachweisbar ein riickgebildetes sechstes Ruderfusspaar ist" Claus says (Geneal. Grundl., p. 82). It is 

 indeed curious that such a very interesting detail can be proved, yet never has been. In his classical monograph on 

 the free-living Copepnda (Marburg, 1863), when describing the appendages of the body, the genital nobs are not 

 mentioned at all, and in his Lehrbuch (4th Autl., Ismi) the same appendages are passed over in silence. 



