NOTES ON EMBRYOLOGY AND CLASSIFICATION. 423 



mouth, and through it runs the new main axis. This lobe 

 is the PROSTOMiUM, and all the organisms which thus deve- 

 lop a new main axis, oblique to the old main axis, may be 

 called prostomiate. 



I have introduced these considerations relative to the 

 changed structural axes of prostomiate, as compared with 

 telostomiate organisms, in order to make clear what follows 

 relative to ciliated bands and tentacles. 



h. Ciliated hands and tentacles ; identity of these struc- 

 tures in Echinoderms and Vermes witlt, the gill-tentacles of 

 Polyzoa, Brachiopods, and Lamellibranchs ; hypothesis of the 

 Architroch. — To Professor Huxley we owe the first perception 

 of the identity of the ciliated bands of the Pluteus larva with 

 the " wheel apparatus " of the Rotifera. Gegenbaur, in his 

 * Grundzuge,' further showed most ingeniously how two 

 ciliated bands surrounding the embryo, one in front of and 

 the other behind the mouth, could be derived from one single 

 circlet circumscribing the mouth, and how, further, the 

 hinder of the two circlets might be suppressed, leaving only 

 a praeoral circlet, which I have proposed ^ to call uniformly 

 the "velum," whether it appears in Mollusc, Annelid, Rotifer, 

 or Echinoderm. 



It does not require very long consideration, in these days 

 of the triumph of the doctrine of " uniformity of type in the 

 structure of animals,^^ to see that it is probable enough that 

 all the ciliated bands of Invertebrate embryos, and even of 

 adult organisms, can be explained as derivatives of one primi- 

 tive organ. By "ciliated bands," I mean, not secondary and 

 unimportant ciliated tracts, but those strongly marked cili- 

 ated ridges often drawn out into successive tentacula, which 

 are at one time or other dominant organs in the animal pos- 

 sessing them, and which may remain throughout life as 

 chief instruments in the economy. Such ciliated bands are 

 the bands and processes of Echinoderm larvse, the ciliated 

 girdles of many Annelid embryos, the tentacles of Phoronis, 

 and the tentaculated organ of Actinotrocha, the ciliated 

 trumpet of Gephyrseans, the " velum "" of Molluscan em- 

 bryos, the similar apparatus of Rotifera, the crown of ten- 

 tacles of the Polyzoa, the gills and labial tentacles of the 

 Lamellibranchia, and the spiral arms of the Brachiopods. 



All these forms can, it appears to me, be derived from a 

 ciliated girdle, which was developed, in all probability, around 

 the ancestral organism by a specialisation of the ciliated 

 ectoderm, at a time when the organism was telostomiate. 



1 " Development of the Pond-snail." ' Quart. Journ. Mic. Sci.,' 1874. 



