COPEPODS OF THE WOODS HOLE KEGION" 9 



segment, the thorax contains six segments, and the abdomen from 

 one to five segments. Some authors regard the genital segment as 

 part of the abdomen, and a few call everything behind the movable 

 articulation the abdomen, even though it may include the fifth 

 thoracic segment. It is perfectly evident, however, that the same 

 segment can not be a part of the abdomen in one copepod and a part 

 of the thorax in another. For this reason it is much better to adopt 

 the nomenclature of Sars, and call that part of the body in front of 

 the movable articulation the metasome and the part behind the 

 articulation the urosome. The latter always includes the genital 

 segment with the abdomen and may or may not include also the fifth 

 segment. In free-swimming forms the fifth thoracic segment is 

 nearly always separated from the sixth, and the fifth legs that it 

 bears are the most distinctive character of the species. In semipara- 

 sitic and commensal copepods the fifth segment is sometimes free 

 and sometimes fused with the sixth. When free the fifth legs still 

 furnish a very important specific character ; when fused they possess 

 little if any specific value. In the parasitic copepods the fifth seg- 

 ment is usually fused with the sixth, and the fifth legs are very often 

 lacking. Even when present they possess practically no systematic 

 value. 



There are six pairs of appendages on the head, two pairs of 

 antennae, one pair of mandibles, two pairs of maxillae, and one 

 pair of maxillipeds. All modern investigators agree in the names 

 of the first three pairs, but in dealing with the last three pairs there 

 are still some eminent writers who continue to regard them as one 

 pair of maxillae and two pairs of maxillipeds. This mode of naming 

 them originated with Claus, who claimed that the last two pairs of 

 these appendages began development as the endopod and exopod of 

 a single pair, and afterwards became separated. But in one of the 

 last papers published by Claus, entitled " Ueber die Maxillarfiisse 

 der Copepoden," * he proved conclusively that he had been mistaken 

 m this claim, acknowledged his error, and changed the names of the 

 appendages to agree with his new discovery, calling them now two 

 pairs of maxillae and one pair of maxillipeds. Giesbrecht and 

 Hansen had already interpreted them in this manner as a result of 

 their studies upon the development of the free-swimming copepods. 

 The present author established the same nomenclature for the para- 

 sitic forms in 1910, although previous to that time he was an advocate 

 of Claus's original claim and unacquainted with his subsequent cor- 

 rection. There is no excuse whatever for continuing the error, and 

 every reason for adopting the new nomenclature, which accordingly 

 is used in this bulletin. 



* Arb. Zool. Inst. Wien, vol. 11 , pt. 1, pp. 49-63, 1895. 



