4 THE VOYAGE OF H.M.S. CHALLENGEE. 



Determination of the Axis of the Echinoidea. 



Of the attempts to define from the position of the antero-posterior axis of the Echinids 

 the relative position of the different zones, by far the most ingenious is that of Loven.^ 

 Adopting the position of the antero-posterior axis of Echinids first defined by Desor, 

 and subsequently fully elaborated by Cotteau/ he has proposed a most admirable 

 notation to denote the several ambulacral and interambulacral areas, which simplifies to 

 a remarkable degree the comparison of the various types, by making it possible to 

 number conveniently the plates of which these areas are composed, and thus readily to 

 call attention to any modifications they may undergo. While, however, we shall adopt 

 generally the nomenclature of Loven, we wish also to be understood as not agreeing with 

 his conclusions regarding the mode of ascertaining the position of the theoretical axis 

 which he has adopted. 



It will be necessary to give a short resum^ of his method of comparison of the 

 corresponding ambulacral and interambulacral areas. If we place any recent Spa- 

 tangoid with the actinal opening upwards and the odd ambulacrum in the line dividing 

 the petaloid ambulacra symmetrically, and call the left posterior ambulacrum I. ; 

 the left of the anterior pair II., the odd ambulacrum III., the right anterior IV., 

 and the right posterior V., — if we then call the first plates of these ambulacra respec- 

 tively a, h, in the order of their sequence from right to left, we shall, according 

 to him, have the following formula to denote the order of identical ambulacral plates ; 

 the larger plates composed of two primary plates, the sutures of which have disap- 

 peared, with two ambulacral pores, are La, Il.a, III.6, IV.«, Y.b, while the corre- 

 sponding plates of the ambulacra belonging to the formula I.b, Il.b, Ill.a, IV.6, V.o, 

 are smaller and only perforated for a single pore (tentacle). 



By going through the whole group of Echinids, exclusive of the Perischoechinidae, 

 Loven finds that by placing the test in the position indicated, the above formula will 

 always hold good; that is by placing the test in such a way that the madreporic body is on 

 the right of the odd ambulacrum, when placed with the actinostome downward. Calling, 

 again, the interambulacral spaces corresponding to them 1, 2, 3, 4, 5, he finds that the 

 large and small interambulacral plates in the Echinoidea (exclusive of the Clypeastroids 

 and Petalosticha) are arranged round the actinal opening according to the formula, la, 2a, 

 3b, ia, 5a for the small plates, and 16, 26, 3a, 46, 56 for the large plates. Now, Lovdn 

 gives us to understand that this denotes some special quality in the areas I. to V. and 

 1 to 5, as he has numbered them. But this it seems to me is not the fact, as we get 

 an identical formula whether we number each of the five ambulacra and interambulacra 

 in succession I. to V. or 1 to 5. The result will always be the same, as will easily be seen 



' S. Loven, Etudes sur las Echinoidees, Kongl. Svensk. Vetensk. Akad. Handl., xi., No. 7, 1874. 

 ^ G. Cotteau, Note snr la famille des Salenides, Bull. Soc. Gcol. de France, torn, xviii. p. 614, 1861. 



