SENSE ORGANS. 125 



radius just within the radial nerve thickening of the ectoneural 

 system, being only separated from the latter by a thin layer of 

 connective tissue (Fig. 86, 4). These are called Lange's cords 

 after their discoverer. They lie in the outer wall of the peri- 

 haemal canal and are mesodermal in origin. They are said to 

 be exclusively motor in function. Round the mouth there ap- 

 pears to be a more or less complete ring belonging to this system 

 in Asteroids and Ophiuroids, but this is absent or much less 

 developed in Echinoids and Holothurians. The deep oral 

 system is not present in Crinoids, or, if it is present, it lies deep 

 on each side of the water- vascular canal (Fig. 197). 



The apical nervous system is also mesodermal and motor. It 

 has the form of a cord in the dorsal middle line and is developed 

 from the dorsal peritoneum with which it sometimes remains 

 continuous (Asteroids, Fig. 86). It is best developed in Crinoids 

 (p. 283), where it is separate from the peritoneum ; it is not found 

 in Holothurians. 



The sense organs, so far as they are understood, are mainly of 

 a tactile nature. There are the tube-feet, which are all highly 

 sensitive, and in some cases (Crinoids, Ophiuroids, certain tube- 

 feet of Echinoids and Holothurians) exclusively concerned with 

 sensation (and respiration). There is the unpaired tentacle, 

 formed by the projecting end of the radial water-vascular trunk, 

 at the end of the radii ; in the early larvae this is the only 

 tentacular or tube-foot structure present. Lastly there are the 

 circumoral tentacles of Holothurians, which contain prolonga- 

 tions of the water- vascular system and are used for grasping, 

 and the buccal tube-feet of regular Echinoids. In some Crinoids 

 there are special tube-feet near the mouth which may be regarded 

 as circumoral tentacles. The circumoral tufts of Echinoids are 

 respiratory in function and not connected with the water- vascular 

 system. 



Of other tactile structures we must mention the spines which 

 are richly provided with nervous tissue. 



Special organs of sense are not numerous. The pigment spots 

 at the end of the arms of Asteroids are probably visual in 

 function (p. 175), as are also the shining spots on the skin of 

 Diadema (an Echinoid, p. 230) . The ocular plates of Echini have 

 no visual structures on them and are merely perforated by the 

 terminal tentacle of the water- vascular canal. 



