RESPIRATION. NERVOUS SYSTEM. 



277 



poric plate into the body cavity, and is kept in active movement by 

 the cilia which line the body cavity and the pevihreinal canals ; in 

 this way the surface of the internal organs is continually bathed by 

 water. The leaf -like and pinnate ambulacral appendages (ambulacral 

 branchice) of the irregular Sea-urchins are regarded as special organs 

 of respiration, as also are the crecal tubes (dermal braiichia-), which 

 project from the surface of the 

 integument and communicate with 



o 



the body cavity in some regular 

 Sea-urchins and in the Asteridea. 

 These dermal branchiae are dis- 

 tributed in the Asteridea over the 

 whole dorsal surface as simple tubes, 

 and in the Echini they surround 

 the mouth as five pairs of branched 

 tubes. Lastly there are the so-called 

 respiratory trees of Holothurians ; 

 these are two large tree-like branched 

 tubes which open by a common stem 

 into the cloaca. The water which is 

 taken into these organs can be again 

 ejected with great force (fig. 219). 



The nervous system (fig. 220) consists of five principal nerves 

 running down the five rays. These nerves in the Asteridea lie imme- 

 diately beneath the epidermis of the ambulacral groove, external to 

 the radial blood vessel and water vascular trunk : they send off 



numerous fibres to the ambulacral feet, 

 the muscles of the spines, pedicellarise, etc. 

 These ectodermal bands may be looked 

 upon as the central part of the nervous 

 system ("ambulacral brains" of J. Miiller). 

 Near the mouth they divide into two parts, 

 which unite with corresponding branches 

 from the other radial trunks to form a 

 nervous ring containing ganglion cells. 



The tentacle-like arnbulacral feet which 

 in the Asteridea and Ophiuridea are 

 present in simple number at the end of the arms are supposed 

 to have the value of tactile organs. The same significance has 

 been attributed to the tentacles of the Holothurids and to the 

 pencil-like tactile feet of the Spatangidw. Organs resembling eyes 



FIG. 220. Diagram of the nervous sys- 

 tem of a Star-fish. N, The nerve ring 

 connecting the five ambulacral cen- 

 tres. 



Oc 



FIG. 221. Astropccten nitnia- 

 fiacus, end of ray with the eye 

 (Oc) surrounded by spicules 

 (after E. Haeckel). 



