TYPE ECHINODERMA. 589 



The epidermal nervous system consists of a perioesopha- 

 geal ring arid five radial nerves as in other forms, and iu addi- 

 tion fi\e interradial nerves pass from the ring to the tentacles. 

 The system is almost completely separated from the ectoderm, 

 with which in the embryo it is connected, and lies within the 

 tissues of the body, the radial nerves only at their posterior 

 extremities passing through the tissues of the body-wall to fuse 

 with the ectoderm. In accordance with this arrangement an 

 epineural sinus accompanies each radial nerve, though absent 

 from the perioesophageal ring. The muscular nervous sys- 

 tem has an arrangement similar to that of the sinuses, being 

 well defined in connection with the radial nerves, though 

 wanting in the pericesophageal ring. No trace of the aboral 

 nervous system has been discovered in the Holothurians. 



The tentacles serve as tactile organs, and may have also 

 an olfactory function in addition to being respiratory, and in 

 the Syuaptids numerous ciliated depressions with a strong 

 nerve-supply are found upon them. No visual organs occur 

 in the Holothurians, but in the Syuaptids and Elasipoda 

 otocysts occur. In Synapta there are ten of these organs, im- 

 bedded in the connective tissue in the neighborhood of the 

 pericesophageal nerve-ring, one lying on each side of each 

 radial nerve close to its origin from the ring. Each otocyst 

 contains a number of otoliths, and is lined interiorly by cells 

 which are provided with cilia. In Elpidia the number of 

 otocysts is fourteen and in other Elasipoda the number may 

 be much greater, amounting to about thirty-six in a species of 

 Kolga. 



The Synaptids and Molpadids are hermaphrodite, but all 

 other Holothurians are bisexual. The reproductive organs 

 (Fig. 269, o) consist of one, or in some cases two bundles of 

 csecal tubes which are attached to the left side (or on both 

 sides) of the dorsal mesentery, and open by usually a single 

 duct (n) upon the dorsal mediau line, sometimes within the 

 circle of tentacles, but usually behind it. The caeca are lined 

 interiorly with an epithelium from which the ova and sperma- 

 tozoa develop. 



The arrangement of the reproductive organs in the Holothuriaus is de- 

 cidedly different from what occurs in other groups of Echinoderms, and the 



