14 THE INVERTEBRATA 



Often cilia are united into compound organs, which may be conical 

 am, paddle-like membranellae (Fig. 14), or undulating- membranes 

 (Fig. 89). Many protozoa which possess a definite body form are able 

 temporarily to alter it by contractions of the protoplasm stretching 

 the pellicle (metaboly), and in various cases this contractility is 

 localized in fibrils, known as myonemes, situated in the ectoplasm. 



Systems of fibres which ramify from a central mass known as the 

 "motorium" and have been thought to be of the nature of a nervous 

 system have been described in various ciliates ; in some of these cutting 

 the fibres is said to destroy the co-ordination between different sets 

 of ciliary organs. It is possible also that the rhizoplast system of 

 flagellates may have a conducting function. Sefise organs are possessed 

 by various protozoa in the form of specialized flagella and cilia in 

 which the tactile sense is highly developed, and by many of the plant- 

 like flagellates as pigment spots (eye-spots), which may be provided 

 with a lens. In some ciliates chemical discrimination between food 

 particles seems to indicate a sense analogous to taste. 



Internal skeletal structures are found in many members of the 

 phylum. They may be part of the living protoplasm, as the axial fibres 

 of axopodia and the axostyles which lie in the midst of the body of 

 various mastigophora {Trichomonas, Fig. 50; etc.) and probably the 

 central capsules of the Radiolaria, or of dead inorganic matter, as the 

 skeletons of the Radiolaria (Fig. 69). 



The Protozoa present every type of nutrition exhibited by organ- 

 isms, except that of the " prototrophic " bacteria,which perform chemo- 

 synthesis by the use of energy obtained from reactions between in- 

 organic substances. Holophytic nutrition,^ however, is found, among 

 protozoa, only in certain of the Mastigophora (see below). Of the 

 holozoic members of the phylum, some feed by amoeboid action. 

 Usually this can be done at any point of the surface, as in the familiar 

 case oi Amoeba, but most of those flagellates which perform amoeboid 

 ingestion do so in a particular region only. Other protozoa swallow 

 through a permanent mouth. The true mouth is the spot at which 



^ The nutrition of an organism is said to be holophytic when it is effected, as 

 in typical plants, by the building up of complex organic substances from 

 simple inorganic ones by use of the energy of certain of the sun's rays (photo- 

 synthesis). The radiant energy is obtained by means of the green, yellow, or 

 brown structures known as " chromatophores " or " chromoplasts " (e.g. the 

 chloroplasts of green plants). In this mode of nutrition the simple materials 

 of the food are absorbed through the surface of the body. In holozoic nutrition 

 complex organic substances are swallowed through temporary or permanent 

 openings as in the majority of animals. In saprophytic (or saprozoic) nutrition, 

 practised by certain organisms, including among others various parasites, 

 which are in contact with solutions of organic matter, relatively complex 

 carbon compounds are taken, but these are absorbed through the body surface. 

 The modes of nutrition classed under this head vary greatly in the complexity 

 of the substances they require. 



