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THE VOYAGE OF H.M.S. CHALLENGER. 



may start directly from the peristome (b 2 on Fig. 6, b), or come off from one of the large 

 furrows which supply the posterior and the two lateral rays (Aj on Fig. 6, b). The furrow 

 on the right or western side of the disk supplies the radi E and D, as in the species with 

 a radial mouth (Fig. 6, a) ; but it is longer and has a larger curve than in these forms, as 

 the d ray is exactly behind the mouth (Fig. 6, b), and the corresponding posterior furrow 

 thus extends round three-fifths of the disk instead of round but one half of it. The 

 shape of the peristome in a species with interradial mouth is well seen in Actinometra 

 regalis (PI. LXVIII. fig. 1); while the origin of the primary ambulacra is also seen in 

 Actinometra belli, though the peristome of this specimen is so much contracted that the 

 position of the mouth on the disk is only indicated by the point of convergence of the 

 ambulacra (PI. LXIV. fig. 2). 



I was led to think at one time that the situation of the mouth, whether radial or 

 interradial, might serve as a character of specific value. But wider experience has 



Fig. 6. — Diagrams showing the different positions of the mouth in Actinometra ; A, with a radial mouth ; B, with an inter- 

 radial mouth. The dotted lines mark the interambulacral regions of the disk. A lt Aj, Ej, E 2 , the five pairs 



of secondary ambulacra. 



shown me that too much reliance must not be placed upon it. For although the mouth 

 generally has a constant position in any given species, it sometimes happens that some 

 individuals of the species have an interradial mouth and others a radial one. Thus, for 

 example, the mouth is radial in nearly all the specimens of Actinometra pectinata which 

 I have seen, as it almost invariably is in the Solaris-gvou]) ; but it is interradial in the 

 three individuals dredged by the Challenger, one in Torres Strait, and two at Samboangan. 

 On the other hand the mouth is interradial in the examples of Actinometra lineata which 

 were dredged at Bahia, but in another individual which I have seen from Barbados its 

 position is radial. 



There is another character which very commonly presents itself in Actinometra, and 

 seems to be correlated with the excentric position of the mouth. I refer to the very 



