J30 LIFE : OUTLINES OF GENERAL BIOLOGY 



the five teeth, and the marks of spines in between. Perhaps this is 

 the strangest of all modes of animal locomotion. 



The movements of most multicellular animals are effected by 

 means of muscles, and there is an important distinction between the 

 unstripcd or smooth muscle of sluggish animals, such as tapeworms, 

 and the striped or striated muscle of ordinary active animals. It 

 may be explained that unstripcd muscle consists of homogeneous 

 spindle-shaped cells, closely fitting accordingly between one another, 

 each with a single nucleus. Ordinary striped muscle is made up of 

 cross-striped cells much elongated, and usually with many nuclei, 

 which are sometimes situated peripherally, as in man; sometimes 

 embedded in the muscle-substance, as in the frog. A striped muscle- 

 fibre is usually a much-elongated single cell; but in some cases a 

 fibre seems to be due to the longitudinal fusion of a few cells. The 

 most important general fact is that unstripcd muscles are slowly 

 contracting, and are therefore found in sluggish animals, such as 

 Ascidians, and in the slowly moving parts of active animals, as in 

 the walls of the food-canal, the walls of the bladder, the walls of the 

 arteries, and the skin around the roots of the hairs. With these and 

 similar exceptions, the muscle-fibres in ordinary active animals are 

 cross-striped and quickly contracting. Some of the lower animals, 

 such as Turbellarian and Nemertean worms, are aided greatly in 

 their locomotion by superficial cilia. The last occurrence of these 

 as locomotor structures is in newly hatched tadpoles, which, like 

 larval lancclets, are richly ciliated. 



Among unicellular organisms locomotion is effected by flagella, 

 by cilia, by myonemes, or in an amoeboid fashion. A flagellum is a 

 thread of protoplasm, sometimes with an axial filament, and it 

 moves in undulations. A cilium is alternately flexed and straight- 

 ened, as one might bend one's arm at the elbow and elongate it 

 again. It is of interest to notice that among multicellular animals 

 cilia are very common, from the lowest to the highest, except in 

 Nematodes or threadworms, where the abundant presence of 

 chit in probably precludes their development. As has been men- 

 tioned, the Turbellarian and Nemertean worms are covered with 

 cilia, which assist in locomotion; but above the level of these two 

 classes, cilia cease to be locomotor except in larval forms, like the 

 trochosphcres of marine Annelids and Molluscs, and in rare cases 

 like Rotifers, which are regarded by some zoologists as arrested 

 larval forms. Starfishes and some other Echinoderms are peculiar 

 in Ix ing richly provided with external cilia, but these are used for 

 wafting food particles, not for locomotion. The big zoological fact 

 is that above Nemerteans the function of cilia ceases to become 

 locomotor, except in larva?. They become of great internal impor- 

 tance, in diverse ways, for they may line a windpipe, an excretory 

 tube, a female genital duct, and so forth. Myonemes are contractile 



