Here we shall consider on the one hand the nature and arrangement of the proteins making up the muscle machine, and on the other hand the chemical substances and enzymic reactions providing the energy. A vast number of observations has been made on examples of the different phyla, but knowledge is still very incomplete and it is often difficult to generalise, particularly because important differences may turn up between closely related species. It will not be possible to deal in a comparative way with more than a part of the data. In the case of two important types, however, the adductor muscle in molluscs (responsible for the proverbial closing mechanism e.g. in the clam) and the fibrillar muscle of certain flying insects (the fastest and most active muscle known) very thorough investigations have been made. Formulation of detailed suggestions for the modus operandi of these two types has been possible.

It is necessary to say a few words about structure, which is very varied. Setting out from histological and electron-microscope observations, Hanson & Lowy (3) have distinguished three kinds of invertebrate muscle; (a) Striated, showing the A and I bands familiar in vertebrate striated muscle; in invertebrates these are found e.g. in insect muscle and in the phasic adductor of Pecten. (b) Muscles such as those in certain cephalopods and annelids showing ‘double-oblique striation’, which depends on the helical arrangement of thick and thin filaments.