Introduction: Control and Coordination in Multicellular Organisms

A single-celled organism can run its whole life on simple, direct chemistry, but once cells organise into tissues, organs and organ systems, two extra jobs appear: control over each process individually, and coordination so that all the different systems work together to keep the internal environment stable (homeostasis). Plants meet this need with chemical signals and movements such as phototropism (bending toward light), chemotropism and thigmotropism (response to touch). Animals, especially the more advanced vertebrates, gradually built up a second, faster channel alongside chemical signalling -- an electrical nervous system -- and the story of this chapter traces how that electrical system became steadily more complex across animal groups before looking at the fully-developed human version, and then turns to the parallel chemical (endocrine) coordination system.

Sponges (phylum Porifera), the most primitive true animals, show this transition still incomplete: even though different sponge cells specialise for jobs like digestion, respiration and excretion (division of labour), sponges have no tissues, no organs and no proper nervous system at all -- coordination between their cells is achieved without any nerve net. A true, if still very primitive, nervous system first appears one step up the animal tree, in cnidarians such as Hydra, and this chapter picks up the story from there before building all the way to the fully centralised human nervous system, and separately to the human endocrine system.

MSBSHSE's HSC Biology syllabus for this chapter draws on the same ground as the NCERT/CBSE Class 11 chapters on neural control and coordination and on chemical coordination and integration, so the terms used here serve Maharashtra board and CBSE students alike.