506 THE ANATOMY OF INVERTEBRATED ANIMALS. 



which characterizes the Echlnodermata is the fact that the 

 alimentary canal of the Echinop^edium gives rise to an en- 

 terocoele, which again is subdivided into two systems of cav- 

 ities, on? ambulacral and the other peritoneal, and that the 

 mesoblast becomes modified in accordance with the arrange- 

 ment of these systems. The enterocoele may be formed by 

 one diverticulum or by three. In the former case, the first 

 formed becomes subdivided into three, of which one is ante- 

 rior, and t\vo lateral, as in the latter case. The lateral di- 

 verticula give rise to the peritoneal cavity and its lining ; 

 the medium diverticulum is converted into the circular ambu- 

 lacral vessel and its dependencies ; and it is in consequence 

 of the radiating disposition of the latter, and of the nerves 

 and muscles which are related to it, that the Echinoderm pos- 

 sesses so much radial symmetry as it displays. It is clear, 

 therefore, that the radial symmetry of the Echinoderm results 

 from the secondary modification of an animal, which is primi- 

 tively bilaterally symmetrical; and that the apparently radi- 

 ate Echinus^ or Star-fish, is a specially modified " Worm " 

 (using that term in its widest sense), in the same sense as the 

 apparently radiate Coronula\'B> a modified Arthropod. 



Haeckel goes further than this, and supposes that each ray 

 of a Star-fish or Ophiurid, for example, represents a Worm, 

 and that the Echinoderm consists of coalesced vermiform 

 buds, developed in the interior of the Echinopaedium. I 

 must confess my inability to see that this hypothesis is sup- 

 ported b}^ valid reasons. On the contrary, the more closely 

 one compares the structure of the ray of an Echinoderm with 

 the body of any known Annelid, the more difficult does it ap- 

 pear to me to be to find any real likeness between the two. 



In order to find any analogy for the production of the 

 Echinoderm within the Echinopsedium, on the contrary, it ap- 

 pears to me that we must look to the lower, and not to the 

 higher, morphological types. Among the Hydrozoa^ nothing 

 is commoner than the distribution of the functions of life be- 

 tween two distinct zooids, one of which alone develops repro- 

 ductive organs. In the former — the hydrantli — radial sym- 



scopical Society in the same year, I expressed the view that the Eotifera " are 

 the permanent forms of Echinoderm larvte, and hold the same relation to the 

 Echinoderins that the Hydriform Polypi hold to the Medusa^," and that they 

 "connect the Echinoderms with the Nematidi? and the Nematoid "Worms." 

 When they were published, those who did not ignore these views, ridiculed 

 them. Nevertheless, thouQ^h somewhat crudely expressed, I think it will be 

 admitted that they have been substantiallv justified by the progress of knowl- 

 edge during tlie last Quarter of a centurv/ 



