208 TOWARDS THE HUMAN FORM 



Medusae, which were bordered at their opening by a mem- 

 branous ring or velum, and how by embryogenetic acceleration 

 they gave rise to the large Medusae, umbrella-shaped, without 

 this velum— Pelagiae, Rhizostomata, etc. All these Medusae have 

 four planes of symmetry, and we can connect with them certain 

 internal impressions with the same kind of symmetry, called 

 Medusites and Laotira, found in the Cambrian. The relation- 

 ship of Brooksella with the Medusae is less evident. 



However, the hydroid polyps possess another interest 

 for us. 



It was formerly believed that all calcareous poly paries 

 were made by organisms similar to those that build up the 

 coral branches. Only two groups were recognized. The poly- 

 paries, formed of tubes divided into storeys by horizontal 

 plates — the tabulate polyparies — and the drugose polyparies, 

 so called because of their coarser appearance. During the 

 famous voyage of the Challenger, a member of the expedition 

 named Moseley showed that among the smooth group there were 

 polvparies whose bases were constructed by organisms called 

 Alcyonarians, 1 akin to the coral-builders, but that the others, as 

 Dana and Louis Agassiz had recognized, were the work of Polyps 

 of the Hydroid group. I myself have shown 2 how the series of 

 Hydroids with smooth, calcareous tabulate branches, which 

 Moseley classed together as Hydrocorallia, and which comprises 

 Spinipora, Millepora, Allopora, Stylaster, Cryptohelia, etc., 

 leads directly through special groups analogous to those that 

 have produced the Medusae, to the present-day Polyps which 

 build reefs, and to the ordinary Sea-anemones, thus forming 

 together the order of Hexacorallia ; and I have pointed out 

 how the embryogenetical researches of Lacaze Duthiers on 

 these Polyps, and those of Marion on the Alcyonarians, force 

 us to the conclusion that, in spite of their eight tentacles, the 

 Alcyonarians were only Hexacorallia modified by embryo- 

 genetic acceleration. Now the Hexacorallia did not make 

 their appearance before the Triassic of the Secondary Period, 

 so that it is not very likely that Heliolites and Plasmopora of 

 the Silurian, and Cladochonus and Syringopora of the 

 Carboniferous, could have been Alcyonarians. They are far 

 more likely to have represented special types of hydrocorallines, 



1 Heliopora. 2 XXVII, 298. 



