546 COMPARATIVE ANATOMY CHAP. 



alone, perhaps, form an exception to this rule, "showing decided resemblance to 

 the Crinoids on the one hand, with an oecasional possible approach to the opposite 

 extreme, i.e. the Holothurioidea, on the other. It is, however, very difficult to judge 

 of the Cystidea, since conclusions as to the inner organisation drawn exclusively 

 from the structure of the skeleton cannot be regarded as altogether trustworthy. 



It appears to us that there is not the least justification for deducing the different 

 Echinoderm classes in any definite way from one another, nor can we at all accept 

 the recently urged view that the Holothurioidea stand nearest to the racial form. 

 On the contrary, the morphology of the genital organs leads us to believe that the 

 Holothurioidea are distinct from all other Echinoderms, with the possible exception 

 of the Cystidea. 



If we review the whole morphology of the Echinodermata, our phylogenetic specu- 

 lations are, first of all, influenced by the fundamental fact that the radiate, but at 

 the same time asymmetrical Echinoderm proceeds ontogenetically from a bi- 

 laterally symmetrical larva, the so-called Dipleurula. 



The Dipleurula Larva. 



This larva is regarded from two opposite points of view. (1) The bilateral 

 structure is thought to have been secondarily acquired, within the different groups 

 of the Echinoderms, in adaptation to the free-swimming manner of life. (2) The 

 bilateral structure of the larva has been inherited from the common racial form 

 of the Echinodermata, or from the larva of such a form. The first view is now 

 generally abandoned. The manner of life might indeed have called forth external 

 bilateral symmetry of form, but certainly not the marked bilateral symmetry of 

 structure of the internal organs. 



If we now try to sketch a hypothetical phylogenetic stage based upon a com- 

 parison of the various Dipleurula larva* of the Echinodermata, the following is the 

 result : The body was freely movable, ovoid, and bilaterally symmetrical ; the mouth 

 lay anteriorly on the ventral side, the anus at the posterior end, or posteriorly on 

 the ventral side. In the frontal region there was a nerve centre below the surface 

 of the ectodermal epithelium which was differentiated into a sensory organ (neural 

 plate). Running back from the nerve centre along the ventral side, below the surface 

 of the body epithelium, were two nerve trunks beset with ganglion cells. The intes- 

 tine was divided into the ectodermal(?) oesophagus, the wider endodermal mid-gut, 

 and the hind-gut, which was also endodermal. At the sides of the intestine were 

 two pairs of coelomic vesicles, the anterior pairs at the sides of the (esophagus, the 

 posterior at the sides of the mid- and hind-guts. The two anterior coelomic vesicles 

 (or their posterior portions) were connected with the exterior by a canal laterally or 

 dorsally (cf. the interesting temporary occurrence of a hydropore on the right side in 

 Asteroids, especially in Asterias vwlgaris, p. 527). The genital products developed 

 out of the endothelium of the coelom. 



Such an organisation has nothing strange about it. It has almost as much claim 

 to be classed with the Vermes as Sagiita has, for which latter classification, however, 

 not much can be said. It is further possible that the racial form possessed special 

 organs for locomotion, respiration, etc., about which nothing can now be positively 

 affirmed, since they have in all cases disappeared from the ontogeny of the Echinoderms. 

 It is not at all likely that the ciliated rings have any phylogenetic significance. 



