KONGL. SV. VET. AKADEMIENS IIANDL. 13AND. 19. N:0 7. 5 



As far as the skeleton of the Echirioidea has beeii subjected to a souiewhat ac- 

 curatu and detailed iiivestigatiou, it has iiivariably comc to liglit that it is in no wise 

 to be regarded as a radiate structurc in the sense of the Cuvierian System, but that 

 its constituent elements are, in reality and fundamentally, arranged bilaterally and 

 symmetrically on the two sides of a mesial plane indicated by its antero-posterior 

 axis. The completely bilateral structure of the larva is in reality never totally dis- 

 carded in the adult, though profoundly obscured. With the bilateral conformation in 

 the adult we have long been familiar, manifest as it is in the låter forms of the Ar- 

 chaionomous^ type, for instance in the Clypeastrida;, of Cenozoie origin, and in the whole 

 of the Neonomous ^) type, as in the Spatangidte, known to us from the Cretaceous period 

 and onward, and in the other forms called Irregular, that come into view during the 

 Mesozoic time. But when we trace back the Echinoidean type into still older periods, 

 those manifestly symmetrical forms are löst, and the class is represented solely by the 

 Cidarida? and otliers, in which the perfectly circular ambitus of the test and the appa- 

 rent similarity of all the live ambulacra as also of the interradial areas of the peri- 

 some to one another, are such as seemingly to exclude any idea of their skeletal ele- 

 ments being subject to bilateral symmetry. Nevertheless, and whatever may have been 

 the case with their predecessors, the little known Perischo-echinidLe, a closer inspec- 

 tion reveals in the antique Cidarida? and their allies a bilateral mode of conformation 

 concealed beneath the deceptive appearance of radiate disposition, exactly identical 

 with that plainly in force in all the rest of the Echinoidea, recent as well as fossil, 

 at present known. ^) This is another instance of the validity of one of the laws more 

 than once ascertained to underlie evolution, namely that structures which are to be 

 gradually, but forcibly worked out during the course of geological ages into specialised 

 and highly characteristic features, are virtually present within the fabric of the earlier 

 forms, though dormant and, as it were, lying in abeyance, and to be detected only by 

 a close scrutiny. Such is the case with the antero-posterior axis of the Echinoidean 

 skeleton. In the Exocyclic it manifestly divides lengthwise one of the tive ambulacra 

 and the opposite interradial area, and the stomo-proctic axis lies in its plane. In 

 most of these forms the ambulacrum which thus becomes the frontal one, III, at its 

 aboral, that is dorsal termination has on its right the one among the ossicles of the 

 calycinal system which in the adult is permeated by the madreporic apparatus, or 

 through which, in the young, this begins to break out from the interiör. Now in the 

 Endocyclic forms, of antique origin, which have the stomo-proctic axis vertical and 

 ending dorsally with piercing in its middle or sideways the dismembered central ossicle, 

 the madreporal filter is invariably and permanently restricted to one of the calycine 

 ossicles alone. To any one believing in the consistency of nature's ways, there is no 

 reason whatever for doubting that this ossicle is strictly homologous to the one which, 

 in the Exocyclic, harbours the madreporite. Acting upon this legitimate supposition 

 Desor and Cotteau in their important Avorks, differing in this point from Johannes 

 Muller, and from Louis Agassiz, invariably adjusted conformably to it all the forms 



') u4QXai6voi.iog, old-fashioned. ^) Nsög, uew, v6i.iog, custom, law. ^) Études, p. 11 — 46. 



