194 



ZOOLOGY. 



to the rabbit, or any other of the vegetarian land -animals, 

 are entirely wanting in the sea. But in the sea (and to a 

 less extent in fresh water) we have conditions eminently 

 suited to the rapid growth and multiplication of microscopic 

 unicellular plants, and these swarm at and near the surface 

 of the ocean. It is on such plants, and the smallest animals 

 that prey upon them, that Amphioxus feeds. Its mouth- 

 region is richly ciliated, and the cilia vibrate in such 

 a way as to direct a constant current of sea-water into 

 the mouth. Carried along helplessly in this current are 



the minute organisms that form its 

 food. Evidently it has little need 

 to wander in search of such food, 

 and so it is not surprising that 

 Amphixous should spend much of 

 its life buried in the sand, the 

 mouth-region alone projecting. It 

 does, however, also swim about 

 actively at times, and by means of 

 its pointed anterior end it can 

 burrow into the sand with great 

 rapidity. 



This mode of feeding, on the 

 minute organisms that can be 

 wafted to the mouth by a ciliary 

 current, is a widespread one 

 throughout the animal kingdom. 

 It is characteristic of the simplest 

 members in almost all the great 

 groups of animals. Thanks to it, 

 many animals, such as the corals, 

 are able to live fixed to the sea- 

 bottom and to grow and branch 

 l^e plants. So enormously abun- 



dant is this food Supply, that it 



really is the ultimate support of 

 almost all the animal life in the ocean, where all the larger 

 animals are carnivorous. 



(From Willey, after Rathke.) 



3. The Atrium. If we proceed to dissect an Amphioxus, 



