540 INVERTEBRATE MORPHOLOGY. 



consists of a ring situated at the aboral surface of the body, 

 sending off branches to the reproductive organs as well as, in 

 some cases at least, forming anastomoses with the epidermal 

 system. 



Sense-organs of various kinds are developed. Tactile 

 tentacles occur at the extremities of the radii of some forms 

 and round the mouth in others, while in the softer-skinned 

 Holothurians tactile papillae may occur. Eyes occur at the 

 extremities of the radial nerves of the Starfishes, and have 

 also been described as occurring in some Echinoids, while 

 otocysts occur in some Holothurians, sometimes in considera- 

 ble numbers. 



No special excretory organs occur in the Echiuodermata, 

 the amoaboid cells of the coelomic fluids perhaps serving in 

 some cases to remove the waste substances. They have been 

 observed to pass through the body-wall, in regions where it is 

 thin, to the exterior and there degenerate. For the most 

 part, however, the waste products are deposited in the tissues, 

 or else pass to the exterior by osmosis. In the Holothurians 

 special branched appendages of the terminal portion of the 

 intestine appear to take some part in excretion, but such 

 organs do not occur in other groups. 



The Echinoderms are almost invariably bisexual, and the 

 reproductive organs are usually situated in the interradii. 

 They are enclosed in a special coalomic sinus, the genital sinus, 

 in whose wall may be found the branches of the aboral nerves. 

 From each organ or mass of reproductive cells a cellular cord, 

 the genital rachis, surrounded by the sinus may be traced, 

 except in the Holothurians, to the ovoid gland, and it appears 

 probable that in some cases at least the reproductive cells 

 originate in a part of the ovoid gland and migrate to the 

 reproductive organ along the rachis, becoming mature in their 

 final position. The openings by which the reproductive ele- 

 ments pass to the exterior vary both in number and position 

 in the different groups, but are usually situated on the aboral 

 surface of the body. 



