£g HOMOLOGIES OF ECHIXODERMS. 



a different interpretation of the Echinodermoidal homologies. Taken 

 in connection with our knowledge of the hard parts of Starfishes as 

 compared with those of Ophiurans and Echini, they throw much light 

 upon many imperfectly known structural features of Crinoids. The liv- 

 ing Crinoids, on account of their small number, have till recently seemed 

 to promise but little help in explaining the fossil forms. The collec- 

 tions of the Challenger include, however, a variety of stalked Crinoids, 

 and until the information to be derived from them is on hand it does 

 not seem advisable to extend the comparison of the hard parts beyond 

 the more common orders. 



All young Echinoderms, while still in the Pluteus stage, or soon after 

 its resorption, are strikingly alike. They all have an actinal and an 

 abactinal area. The actinal area is occupied almost entirely by the 

 ambulacral canals radiating from the central ring enclosing the actino- 

 stome, with their lateral ambulacral tubes existing as mere loops, the 

 different tubes not being as yet encased in any limestone plates. The 

 abactinal surface consists of the outer integument, in which rudiment- 

 ary plates begin to appear, made up in the early stages merely of Y- 

 shaped rods, more or less closely connected together, so as to form 

 patches of reticulated network to become in the future the solid plates 

 of the Echinoderm. 



Thus far all Echinoderms are alike, and show no structural difference 

 between the different orders. We shall greatly facilitate our examina- 

 tion of them by beginning our comparison at this early and uniform 

 stage, so that we may see how far we can, by merely tracing the devel- 

 opment, explain the mode of differentiation by which the orders gradu- 

 ally assume the structural features of the adults. 



Before proceeding any further in this comparison, I must state that 

 I have given already in detail*" my reasons for considering the Echi- 

 noderms as more closely related to the Polyps and Acalephs in oppo- 

 sition to the view lately revived of their affinity to Worms. I liave 

 stated the objections mainly on embryological grounds, by comparing 

 the development of the most Echinodermoid larva among Annulata, that 

 of Balanoglossus, with other vermiform Echinoderm Plutei. Ilacckel has 

 recently strongly urged, on theoretical grounds chiefly, their annulate 



• Sec A. Agassiz, Embryology of CtcnoplioraB ; Revision of the Echini ; The History of Bananoglossus 

 a:i I Tornaria. 



