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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1891 Excerpt:...and systems of organs to the outer integument are very varied. These relations are closest in the nervous system in consequence of its dependence on the (ectodermal) sensory organs. As already described (cf. p. 223), in many worms the central nervous system remains in the integument, even in the adult animal, and is thus not mesodermal. The mesodermal position of the nervous system is, however, the general rule, as it is even as early as in the Platodes. As, however, it nearly always develops entirely separate from the rest of the mesoderm, we shall describe its development first. The Brain.--In some cases the brain, or a part of it at any rate, develops out of the ectodermal neural plate (many Annelids, Sipuneulidm). The elements of the neural plate probably arise in connection with the provisional or definitive sensory organs of the head (eyes, neural tuft, tentacles), though this cannot be established in detail. There are thus different parts which unite together to constitute it a sensory nervous centre. The neural plate must thus represent an organ similar to the sensory body of the Ctenophora or the marginal centres of the Medusa;. It is often thrown off with the larval integument (e.g. in Phoronis, Pilidium), and the oral nerve ring or the brain arises anew out of the secondary ectoderm. Ventral Chord of the Annulata.--The ventral chord seems always to begin to form separately from the neural plate. It arises either as a continuous thickening of the ectoderm in the ventral middle line, or as a pair of thickenings one on each side of this middle line. The differentiation of the rudiment of the ventral chord into the definitive ventral chord goes hand in hand with the development of the rest of the trunk, and proceeds from before backward. It either re...