LESSONS IN ZOOLOGY. 263 



ered with numerous tubercles, to which spines of varying lengths 

 are jointed. The mouth on the underside is armed with a com- 

 plicated apparatus of calcareous teeth. From the stomach pro- 

 ceeds a long convoluted intestine, attached to the interior of the 

 shell by a delicate membrane or mesentery. The surface of the 

 mesentery and the lining membrane of the shell are furnished with 

 cilia, and by their motion the fluids of the body cavity are kept 

 in circulation, and in this way respiration is effected, and, in some 

 cases, they are provided with branched respiratory tubes. The 

 reproduction is sexual, the fertilized ova forming ciliated free 

 swimming embryos which pass through a complicated develop- 

 ment. 



The Holothurians or sea-cucumbers are worm-shaped animals 

 with a leathery skin, endowed with longitudinal and transverse 

 muscles, by which, when irritated, some species can break their 

 bodies into several pieces. The mouth is surrounded by a circlet 

 of feathery tentacles. Locomotion is accomplished by the alter- 

 nate extension and contraction of their worm-like bodies. They 

 sometimes attain considerable size, a,nd are highly prized as food 

 by the Chinese. 



Vermes or Worms. The worms are bilateral animals with a 

 much elongated, flat or cylindrical body without appendages. 

 Just beneath the skin there is a strongly developed muscular 

 system. The nervous system consists of a single ganglion, or a 

 pair of ganglia situated toward the anterior of the body above 

 the gullet, and sometimes of a nerve ring around the gullet. The 

 nerves given off are distributed symmetrically, supplying the sense 

 organs and forming two strong lateral nerves. The sense or- 

 gans are eyes or eye spots, auditory apparatus and tactile cor- 

 puscles. The function of respiration is performed through the 

 surface of the body. Sexual reproduction is common, but gem- 

 mation and fission often occur. 



The Tubellaria are worms that have a small flattened body 

 covered with cilia. They occur in salt or fresh water, and lead 

 an independent life. 



The Trematoda are suctorial worms or flukes, and are all in- 



