38 W. PERCY SLADEN. 



ia general. It may be said to be universally admitted that the 

 interradius with which the water-pore or madreporite is associ- 

 ated stands in a definite and fixed relation to the internal 

 organisation. This relation is constant throughout the group. 

 It is natural, therefore, that the madreporite has always been 

 regarded as an important superficial index, whereby the orien- 

 tation of an Echinoderm may be eff'ected. Taking these 

 statements as a premiss, I would now refer to Loven's classical 

 memoir on the structure of the Echinoidea,^ in which the 

 bilateral symmetry of all Echinoids, both regular and irregular, 

 is most clearly established. The madreporite is here shown to 

 be invariably associated with the right anterior interradius, 

 when the test is viewed from above. If Loven's demonstra- 

 tions are accepted as conclusive (and I myself admit them and 

 consider that the evidence in its general bearings is as yet 

 unshaken ^), it necessarily follows that as the position of the 

 madreporite is fixed and definite in its relation to the internal 

 anatomy of the organism, and consequently to the antero- 

 posterior axis, throughout all the group, the axis of bilateral 

 symmetry is the same iu each, and the madreporite holds the 

 same definite relation thereto as predicated above. On these 

 grounds I regard the interradius of the madreporite or water- 

 pore of the Crinoid, the Ophiurid, and the Asterid, as the right 

 anterior one when viewed from the dorsal surface ; and in con- 

 formity with this opinion I have thus placed the diagrams of 

 the respective larvae referred to in the preceding pages. It 

 seems unnecessary that I should here enumerate the various 

 diverse views which have been held by other writers upon the 

 subject. 



Whilst treating of the madreporite I am naturally led to 

 remark on a recent paper by my friend M. Perrier^ on 



' " Etudes sur les Echiaoidees," ' Kongl. Vet. Akad. Haudl.,' Bd. xi, No. 7. 



- Although I have very carefully studied Prof. A. Agassiz' discussiou of 

 this question in his ' llevision of the Echiui' and iu his 'Report on the 

 " Challenger " Echiuoidea/ I do not yet feel disposed to relinquish my adhesion 

 to the opinions above expressed, in so far as the significance of the test struc- 

 tures, when taken as a vphole, is concerned. 



3 'Comptes Rendus,' 10 July, 1SS2, t. xcv, pp. CI -63. 



