244 THE FOOD OF ANIMALS 



elsewhere (vol. i, p. 49) been described as microscopic threads of 

 living matter, which closely beset certain parts in many animals. 

 They may in lower forms cover the external surface, and they 

 also clothe the linings of some internal cavities. They are pos- 

 sessed of the power of spontaneous movement, each individual 

 cilium alternately bending and straightening. As numerous cilia 

 bend in the same direction at the same time, those possessed by 

 aquatic animals often set up definite currents in the surrounding 

 water. Now this water contains a great deal of available food, 

 in the form of minute particles of organic matter of varied origin, 

 together with microscopic animals and plants. As a result the 

 currents in question often play the part of food-bearers, bringing 

 an abundance of mixed diet within reach of the animals which 

 originate them. And many forms of life which are richly provided 

 with these motile threads depend entirely upon ciliary currents 

 for their livelihood. 



LANCELETS (CEPHALOCHORDA) 



The highest Protochordate is a small somewhat fish-like 

 form, commonly known on account of its shape as the Lancelet 

 (Amphioxus} (see vol. i, pp. 293297). Like all known Proto- 

 chordates it is exclusively marine, and is found on the coasts 

 of many countries situated in widely - separated parts of the 

 world. It is not unknown, though far from abundant, in British 

 seas. Entirely devoid of jaws, its digestive tube commences in 

 a widely-open funnel-shaped cavity, which communicates behind 

 with a large pharynx, the sides of which are perforated with 

 numerous oblique gill-slits. These open externally into an 

 atrial cavity, that again communicates with the exterior by a 

 small pore placed on the under side of the body pretty far 

 back (see vol. i, p. 294). The interior of both mouth -funnel 

 and pharynx is richly provided with cilia, and these work in 

 such a way that currents of sea - water continually enter the 

 mouth, the particles of food which they bring being conducted 

 back to the stomach and intestine. The water which brings them 

 also serves for breathing purposes, and passing out through the 

 gill-slits ultimately reaches the outside of the body. The food- 

 particles, however, are prevented from doing this, for they are 

 entangled in a sticky substance and conducted back by means of 



