xvi ECHINODEKMATA 561 



of the ancestor were in every way similar to the lophophoral 

 arms of Brachiopoda, to the halves of the lophophore of Polyzoa and 

 of Phoronis, and performed a similar function ; and it must be 

 remembered that all these lophophores consist of hollow protrusions 

 of a special division of the coeloni, clothed externally with ciliated 

 epithelium. The peculiarity of the Echinoderm ancestor was the 

 anterior coelom with its dorsal ciliated pore ; this, when both 

 hydrocoeles were equally developed, was also probably double. 



No adult animal exactly like this supposititious ancestor at 

 present swims in the seas, but there are many features of resemblance 

 between the Dipleurula, whose many features have thus been re- 

 constructed, and a simple Ctenophore, if we regard the gastro-vascular 

 canal system as representing a coelom which is still in open com- 

 munication with the digestive gut, represented in Ctenophores by the 

 infundibulum or funnel. In Ctenophores the gastro-vascular canal 

 system consists of an anterior section which is divided into a series of 

 lobes underlying the ctenophoral canals, and which gives rise to the 

 canal which leads to the base of the tentacle, and of a posterior section, 

 the paragastric canal, which runs backwards to the posterior end of 

 the animal ; there is also an anterior unpaired canal given off from the 

 upper end of the funnel which terminates in two pores at the anterior 

 end of the animal. The primitive mouth is not yet divided into mouth 

 and arms, so that the Ctenophore represents a more primitive con- 

 dition than our hypothetical Dipleurula. Locomotion is effected by 

 radiating bands of ciliated epithelium ; and where they converge at 

 the apical pole there is situated a median neuro-sensory patch of 

 ectoderm, which is in every way comparable to the organ which 

 appears in the Echinopluteus larva when it is three weeks old, and 

 to the sense-organ at the apex of the Crinoid larva. 



Now the arrangement of the ciliated ectoderm is very different 

 in a Ctenophore from what it must have been in a Dipleurula ; still, 

 if we had before us the Dipleurula and the Ctenophore-like ancestor 

 of Annelida and Mollusca, when the mouth and arms had become 

 separated from one another, we should probably have regarded them 

 as members of the same class of animals. We must remember the 

 marked tendency of patches of ciliated epithelium to undergo 

 rearrangement and reunion as is shown by the production of 

 transverse ciliated bands in the Auricularia, and in the Crinoid 

 larva, and in the Echinopluteus of Echinus esculentus, and these 

 transverse bands do not exactly correspond to one another in any 

 of these three cases. 



It is a fascinating and in every way a likely supposition, that 

 there existed a class of simple marine animals, distributed all over 

 the world and diversified into orders, genera, and species, and that 

 the Ctenophore-like ancestor of Annelida and Mollusca was one 

 member, and the Dipleurula another member of this class. 



If, however, we may draw conclusions from the representations of 

 these ancestors given us in larval forms, i.e. in the larvae of Echiuo- 



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