214 Inter-relations of the Classes of the Echinodermata. 



the generative organs are radial in position or have undergone 

 fusion and become interradial. 



The Statozoa are actinogonidiate caliculate Echinodermata 

 in which the oral surface of the body looks upwards, the body- 

 is temporarily or permanently fixed, the podia have a respira- 

 tory function only, and the anus opens on the oral surface. 

 They may (pelmatozoic) or may not (apelmatozoic) have or 

 have had a stalk. 



The Holothuroidea are non-caliculate, anactinogonidial, 

 apelmatozoic Echinoderms, in which the skeletal system is 

 spicular or irregular ; the musculature of the body-wall is 

 well developed, and the mouth is surrounded by a circlet of 

 never very numerous tentacles communicating with the water- 

 vascular system ; this is or is not provided with podia. The 

 mouth and anus are at or near the opposite ends of a generally 

 elongated body. A few are hermaphrodite and a few have 

 been found in brackisli water. 



The Eleutherozoa are actinogonidiate caliculate Echino- 

 dermata in which the oral surface of the body looks down- 

 wards, the power of locomotion is retained, and the podia are 

 often locomotor in function ; the anus, if present, varies in 

 position. 



The Zygopoda are Eleutherozoa in which the podia extend 

 more or less uninterruptedly from the calycinal to the oral 

 region. 



The Azygopoda are Eleutherozoa in which the podia are 

 all or nearly all on the oral surface of the body only, and are 

 separated by terminal plates from any contact with the caly- 

 cinal area. 



The Echinoidea are caliculate, actinogonidial, eleutherozoic, 

 zygopodous Echinoderms, in which the calycinal area may 

 be very extensive, reduced, or greatly metamorphosed ; the 

 gonads are unpaired and interradial ; the body is perfectly 

 rounded, more or less flattened, or bilaterally symmetrical, 

 and is more or less covered by spines which may be long, 

 stout, and strong, or present every stage of reduction to such 

 as are fine and silky. They are all pioctuchous, but the anus 

 is not always opposite the mouth. Respiration partly by 

 gills and partly by the podia, which may be specially 

 modified. 



The Asteroidea are caliculate, actinogonidial, eleutherozoic, 

 azygopodous Echinoderms, in which there is an open ambu- 

 lacral groove. 'Jhe stellate form of the body is often well 

 marked and the rays prolonged into " arms," which vary in 

 their proportional length to the diameter of the disk. The 

 digestive system, which is rarely aproctous, and the genera- 



