AMONG THE SEA URCHINS. 267 



flesh, within which is a large oval cavity " — the stomach (s.), which, 

 singularly, is the only part of the larval form not absorbed and 

 lost in the curious development which soon takes place. The size 

 of the creature is very small ; only as large as an ordinary 

 period (.). The mouth is seen at m., the anus at a., while w.p. 

 is the external water-pore-opening, leading into the water vascular 

 ring, w. This tiny creature is beautifully ciliated, especially 

 down the legs, as we may call them, and on two "epaulettes," 

 one on either side, just above the anus and water-pore in 

 the woodcut, and close to the figure 5 in the plate, which, 

 being drawn from one of my own microscopic mounts, does not 

 show them as in life. Dr. Carpenter, in his work on the 

 Microscope (last edition, p. 646), gives excellent directions as to the 

 best method of mounting these larvae, quoting the authority of 

 ]\Ir. Percy Sladen, Secretary of the Linnean Society. No mount, 

 however, can give the beholder an idea of the regular, easy, and 

 beautiful motion with which these minute forms of life thread 

 their way through the mighty waters of the trackless deep, without 

 compass or guide, until the instinct within them warns them to 

 seek a lower and safer level for further development in their life 

 history. 



Space forbids much reference to the family affinities of the 

 Sea Urchins. Agassiz considers there is a close structural rela- 

 tionship between the Echinoderms and the Ctenophorse (Cydippe, 

 etc.), while there is an idea, originated by Haeckel and lately 

 strongly supported by Gegenbaur, that the Sea Urchins are com- 

 posite creatures, being, in fact, colonies of worms. There is 

 certainly a curious resemblance in the larval development of some 

 of the Gephyrean worms and those of the Holothurian and Star- 

 fish Echinoderms. 



There is, however, this important difference, that the worm- 

 larva becomes transformed^ by successive moulting, into a worm ; 

 while the Echinoderm larva is absorbed and eaten ujj, as it were, 

 by the growing young Sea Urchin, etc. 



We may, however, be certain that in these larval similarities 

 we find the true evolutionary missing links between the otherwise 

 greatly divided families with which the Creator has peopled our 

 globe. 



