AFFINITIES : PHYLOGENY. 165 



between the Echinodermata and the Chordata and we have 

 seen that they all, except possibly the last, are concerned with 

 fundamental, not superficial, traits. Some of them link the 

 Echinoderms to all the Chordates or to all except the Tunicata, 

 e.g. the relations of the central nervous system and the general 

 relations of the coelomic sacs ; another they hold in common 

 only with Vertebrata, viz. the presence of calcified plates in the 

 mesoderm of the body wall ; a third the sinistral position of 

 the mouth is found again only in Amphioxus ; and lastly 

 there is the striking resemblance to Enteropneusta by the 

 tornaria larva. The accumulated weight of these facts is over- 

 whelming and leaves us no choice but to consider not only that 

 the Chordata are the nearest allies of the Echinodermata, but 

 that the Echinodermata are of all Coelomata the nearest to the 

 Chordata. 



FIG. 116Ms. diagrammatic representation of the supposed Dipleurula ancestor of Echino- 

 derms, seen from the left side, with the ventral surface towards the substratum (after 

 Bather). 1 right and left water-pore ; 2 preoral lobe ; 3 nerve plate of preoral lobe ; 4 

 anterior coelom of the left side ; 5 mouth ; 6 left posterior coelom ; 7 anus ; 8 right 

 posterior coelom ; 9 right hydrocoel ; 10 left hydrocoel. 



We now come to a consideration of the so-called Dipleurula, a hypothe- 

 tical form which has been imagined by some zoologists as the bilateral 

 ancestor of all Echinoderms. The structure of this hypothetical animal 

 will be understood at a glance from an inspection of Fig. llQbis, which 

 represents it as seen from the left side with its ventral surface turned 

 towards the substratum. It is a bilaterally symmetrical animal with 

 a preoral lobe (2) carrying a nervous plate (3), a ventral mouth (5), a 

 tarminal or ventral anus (7), three coelomic vesicles on each side (4, 

 10, 6), and two water-pores (1). How has this form been arrived at ? 



It is not arrived at by selecting features common to all the free-swimming 

 bilateral larvae (sometimes called Dipleurula larvae) of Echinoderms ; 

 but by picking and choosing from among the characters of different larvae 

 those which, according to the preconceived ideas of its authors, the common 

 ancestor might be supposed to have possessed, and adding one or two 

 characters which none of them possess. For instance, the preoral lobe is 

 very small in pluteus larvae, though well developed in bipinnaria and 

 in the larva of Crinoids ; a preoral nervous system has been detected 



