Professor Huxley divides the vertebrates into ITHYCOIDS, comprising fishes and amphibia, which, besides other characteristics, have gills at some period of their existence; SAUROIDS (reptiles and birds), which have no gills, and possess certain developmental characteristics in common; and, lastly, MAMMALS. The Insecta, Myriopoda, Arachnidæ, and Crustacea, he remarks, "without doubt present so many characters in common as to form a very natural assemblage. All are provided with articulated limbs attached to a segmented body skeleton, the latter, like the skeleton of the limbs, being an 'exoskeleton,' or a bordering of that layer which corresponds with the outer part of the vertebrates. In others, at any rate in the embryonic condition, the nervous system is composed of a double chain of ganglia, united by longitudinal commissures, and the gullet passed between two of these commissures. No one of the members of these four classes is known to possess vibratile cilia. The great majority of these animals have a distinct heart, provided with valvular apertures, which are in communication with a peri-visceral cavity containing corpusculated blood." These four classes have constituted the larger group or "province" of _Articulata_ or _Arthropoda_. Professor Huxley thinks that, notwithstanding "the marked differences" between the Annelida (worms) and the preceding Arthropods (joint-foots), their resemblances outweighing them--"the characters of the nervous system, and the frequently segmented body, with imperfect lateral appendages of the Annelida, necessitates their assemblage with the Arthropoda in one great division, or sub-kingdom, of ANNULOSA." Tracing analogies between the Echinodermata (sea urchins, star-fish, &c.) and the Scolecida (intestinal worms), he places them together as _Annuloida_. Cephalopoda, Pteropoda, Pulmo-gasteropoda, and Branchio-gasteropoda, having resemblances of nervous system, and "all possessing that remarkable buccal apparatus, the Odontophore," are placed together by him as ODONTOPHORA. The Odontophores (tooth-bearers) are familiar to microscopists as the so-called _palates_ of mollusca. Placing with the above the lamellibranchial mollusks (mollusks with gills formed of lamellæ or little plates), Ascidioida (ascidians), Brachiopoda (lamp-sheds), and Polyzoa, in spite of their differences, he forms another great group, ANNULOIDA.