IÓI 



As has already been said, I propose to make use of the structure of the 3 rd and 

 4 th pairs of the cirri for the classification of the species of the genus Balanus. I think that 

 Fritz Muller was the first to observe that such claw-like spines occurred on the cirri of a 

 species of the genus Balanus (B. armatus): according to him, they are in that species strongly 

 developed on the segments of the 3 rd , and on the posterior ramus of the 4"' pair, but, in 

 less developed condition, on some segments of the 5 th and 6 th cirrus also. On the same 

 occasion he recorded the presence of such spines on the cirri of other species of Balanus, 

 of which he gave B. improvisus, var. assimilis as an example. Recently (191 1), Krüger 

 pointed out a similar peculiarity in the cirrus of the 3 ld pair of B. trigonus Darwin, and in 

 the mean time, the study of the species of Balanus collected by the Siboga has shown that 

 such armature is a common feature of several other species of this genus. In some species 

 it is the 3 rd cirrus only which is armed, in others it is the fourth, and there are also species 

 in which both cirri and sometimes the 2 nd or the 5 th also are furnished with these claw- 

 like teeth. In other respects these species differ more or less considerably from one another, 

 and so it has .not been possible to include all of them in the same section of the genus 

 Balanus. For, while some of these (B. amphitrite for an example) in every respect are 

 true Balani, it cannot be denied that some of these species with armed cirri show resemblance 

 to species of Acasta. Yet even the latter species, when we take all their characteristic features 

 together, cannot be separated from Balanus. They must therefore remain in that genus but at 

 the same time may be considered as forming the link between that genus and Acasta. 



Besides the fact that it is not always the same cirrus (or cirri) which shows (or show) 

 the armature, another difference must be pointed out with regard to the shape of the 

 teeth themselves. They are of two different types : on the cirrus or cirri of some species they 

 appear like small triangular spines, which are attached to the surface of the segment with 

 a rather broad basis and, as a rule, at some distance from each other. The surface of 

 attachment can distinctly be made out as a small place of oval shape, exactly as is the case 

 with other spines, or hairs, which, as a rule, have a rounded surface of attachment. The 

 other type consists of teeth, which are like little hooks slightly curved downwards, or (seldom) 

 upwards, and form so many chitinous excrescences along the margin, which often stands off 

 like a list or rim on the surface of the segment. These teeth are close together, and form 

 a saw-like structure. Transitions from teeth of the one type to the other are, however, not 

 infrequent. 



I gïve here a list of the species of Balanus which I found furnished with armed cirri 

 and which belong to the collection made by H. M. S. "Siboga" : 





Section Ortho-Balanus 



B. amphitrite : 3 rd cirrus, both rami with a row of downward-curving teeth along anterior face 



of numerous segments. One or more parallel rows of smaller teeth on the surface 



of the segments. 



4 th cirrus, both rami have a group of smaller teeth on each of the 1 2 lower segments. 



B. alatus : 3 rd cirrus, the longer ramus with a few teeth between the hairs near the extremity 



33 



SIBOGA-EXPEDITIE XXXI b. 21 



