CALAMOCRINUS DIOMED.E. 91 



gists are jigreed, the dilTerent classes of Ecliinodenns have developed in- 

 dependently from the earliest fossilit'erons times, this very fact presents 

 the greatest dilficnlties we have to enconnter at present in tracing back 

 the earliest history of the Echinoderins ; for, as shown by so many 

 palaeontologists, in the oldest of the fossiliferous beds we already find 

 representatives of the different orders (of course excluding Holothurians) 

 very highly specialized, and bringing us scarcely nearer to the primordial 

 echinodermal type than we are to-day. With our partial knowledge of 

 the fossils of those earlier times, we meet very much the same difficulties 

 in tracing their homologies as in following those of their representatives 

 of to-day. 



May we not look upon the mouth tentacles of Holothurians as some- 

 thing peculiar to themselves, of which perhaps the gills of Echini and 

 the Polian vesicles of Starfishes are the analogues, and not attempt a 

 homology of parts situated radially in these classes of Echinoderms with 

 parts the position of which is doubtful in Holothurians ? 



It is evident that to the pentactinal arrangement of the hard parts of 

 the cal3'x of Echinoderms is due much of the analogous arrangement which 

 we note among them ; but it does not follow, because some of the plates 

 which have been included by Carpenter and others in their homologies 

 of Echinoderms do not belong there, that therefore some plates of the 

 calyx of the different groups of Echinoderms are not to be homologized, 

 and differences of opinion may possibly not be fatal to the value of the 

 homologies. 



I am surprised that Ludwig, after the attempts he has made to reverse 

 the recognized homologies between the hard parts of Echini and Star- 

 fishes, should express such views as the following on the value of the ho- 

 mologies he has endeavored to establish : " Irgend welche unmittelbaren 

 Beziehungen zu den inneren Organen des Thierkorpers sind, bei dieser 

 lediglich auf die raumlicha Anordnungsweise jener Flatten gegriindeten 

 Homoloajisirung nicht in Betracht o-ezo^en worden." He seems to have 



O O DO 



forujotten that he has based somethinu; of the strength of his former ar- 

 gument on the fact which had previously been made known by myself, 

 and later by Goette, of the development of the actinal and abactinal 

 systems from the two opposite water tubes of the pluteus stage. 



We need further proof than that given by Bui'y's observations, which 

 go counter to those of every other embryologist who has studied Echino- 



