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the upper part of each radiating partition, connecting the upper part 

 of adjoining chambers. As we thicken our partitions, we of course 

 change this passage from one chamber to another, which was a mere 

 hole, into a tube of the depth of the wall, and thus form a circular 

 tube which connects the radiating tubes. But as we thicken the 

 walls of our Polyp formula, we separate the bunches of eggs which 

 were before only kept apart by a thin partition, and bring thus 

 together the bunches of two different partitions, exactly as we find 

 them on the sides of the radiating tubes in Acalephs. At the base of 

 the tentacles of Acalephs, placed in the prolongation of the radiating 

 tubes, we find an accumulation of pigment cells, which becomes in 

 some of them quite a complicated eye. At the base of the tentacles 

 of Polyps, we find also a similar accumulation of cells, which, though 

 never becoming a perfect eye as in Acalephs, shows by its position that 

 it is this rudimentary eye which is more developed in the Acalephs. 



When we come to Echinoderms we have a solid envelope, in which 

 we find an alimentary canal, with a distinct wall, winding from one 

 pole to the other, and tubes with a complicated system of suckers 

 running on the inner side of this envelope. 



How can this formula be transformed into that of a Polyp or an 

 Acaleph, and the reverse ? Let us take our Acaleph formula ; instead 

 of the gelatinous mass out of which the tubes and the cavity have 

 been scooped, we shall have a cavity and tubes with distinct walls, if 

 we suddenly condense all this mass, and throw it to the ciix-umference, 

 leaving the tubes and cavity to hang loosely in the envelope which 

 has become solid by the deposition of limestone particles. Let us 

 see how this formula agrees in other respects with the other formulae. 

 The ovaries of the Acalephs and Polyps are placed on each side of 

 the partitions, on what I would call the interambulacral spaces. 

 We find the ovaries of Echinoderms in the space between the 

 tubes, or in the interambulacral spaces. The eyes, which in the 

 Acalephs and Polyps are at the extremity of the radiating tubes, we 

 find in our formula for Echinoderms also at the end of the radiating 

 tubes. And to make the homology complete, there is at the end of 

 the rays in all genuine Starfishes an odd ambulacral sucker, iden- 

 tical in all its organic relations with the tentacles in the prolongation 

 of the radiating tubes of Acalephs. I need not urge that the compli- 

 cation of the radiating tubes of Echinoderms assuming the form of 

 gills, as in many Echinoids, — or of extensively ramified tentacles, as 

 in Holothurians, — does not afiect the homology of the ambulacra 

 with the radiating chambers of Polyps, any more than the presence of 

 protractile ambulacral suckers, since among Holothurians the Synaptce 

 have simple radiating tubes taking the place of ambulacra. 



The three formulae by which we represent Polyps, Acalephs, and 



