ECHINODERMATA SENSORY ORGANS 



467 



C. Auditory Organs, Organs for Orientation. 



Two types of organs for hearing or for orientation have been 

 observed in Echinoderms : (1) the auditory vesicles (Baur's vesicles, 

 otoeysts) of certain Holothurioidea ; and (2) the sphseridia of the 

 Eclrinoidea, which have already been described. 



1. Auditory vesicles are found only in the Holothurioidea, and 

 among these in the Paradinopoda (Synaptidce), and among the Actino- 

 poda in the Elasipoda. 



They have been best observed in the Synaptidce (Fig. 376). In 

 this family, five pairs of these vesicles occur in the cutis of the body 

 wall, near the tentacles, at the points where the five radial nerve 

 trunks emerge from the calcareous ring. On the outer side of each 



7 ff 



FIG. 376. Section tlirough the two auditory vesicles of a radius of Synapta ''after Cuenot). 

 1, Epineural sinus ; 2, epithelial wall of the auditor}* vesicle; 3, otoliths; 4, nervus acusticus ; 

 5, longitudinal muscles ; 6, pseudohaemal canal ; 7, radial nerve trunk. 



nerve trunk lies a pair of otoeysts. Each otocyst consists of a vesicle 

 filled with fluid, with a wall of (ciliated) plate epithelium. Numerous 

 otoliths are found vibrating in the fluid. These otoliths are vesicular 

 cells, the cavity of each being filled by a hard mass (phosphate of 

 lime). The nerves to the otoeysts (nervi acustici) come from the 

 radial nerve trunk. 



The auditory vesicles of the Elasipoda occur in great numbers ; 

 there may be fourteen to one hundred or even more. Xot infrequently 

 their manner of distribution is bilaterally symmetrical. For instance, 

 in Elpidia glacialis, six of the fourteen auditory vesicles occur on the 

 two lateral radii of the trivium, and one on each of the two radii of 

 the bivium. In this case no vesicles occur in the ventral median 

 radius, 



2. The sphaeridia of the EchinoiJea, which are regarded as 



