HOLOTHURIOIDS. 



499 



the internal organs are contained within walls of their 

 own. The muscular system is well developed. The ner- 

 vous system consists of a ring around the commencement 

 of the oesophagus, which sends off branches along the 

 rays. Respiration is performed by means of branchiae, by 

 organs performing other functions, and by water passing 

 into the cavity of the body, and thus aerating the blood 

 through the capillary vessels of the viscera. Echinoderms 

 increase by means of eggs, are marine, and are abundant 

 on almost every coast ; and the remains of extinct species 

 fill the rocks in many regions. It is an interesting fact, 

 that in Radiates there is generally a definite or reigning 

 number to which the parts conform. In Echinoderms 

 this number \sfive; that is, the parts of a given kind are 

 generally five or a multiple of five. They embrace five 

 orders,* Holothurioids, Echinoids, Asterioids, Ophiuri- 

 oids, and Crinoids ; ranking in the order named. 



SUB-SECTION I. 



THE ORDER OF HOLOTHURIOIDS, OR HOLOTHURIANS. 



THE Holothurioids comprise echinoderms which have 

 the body long, cylindrical, somewhat worm-like in gen- 

 Fig. 482. 



Holothurian, or " Sea Cucumber," Pentactafrondosa, North Atlantic. 



eral appearance, with a row of appendages around the 

 oral opening, and without a calcareous shell, but with a 



* Six, according to those who regard Sipunculoids as Echinoderms. 

 Some class them with Worms. Forbes calls them Annelidous Radiates. 



