2 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 41. 



that have no jointed column, but nevertheless are attached, usually 

 by a short thick stalk, are likewise discussed. These Pelmatozoa in 

 a way are to be regarded as intermediate between the statozoic and 

 eleutherozoic forms, as regards their mode of life, and may be dealt 

 with to advantage in the present paper. Material throwing not a 

 little light on certain questions concerning these various types has 

 become available and the evidence thus afforded may prove of some 

 interest. 



There are many well-known eleutherozoic Pelmatozoa, but it seems 

 probable that the assumption of a free or semifree existence has 

 been far more prevalent among these forms than is generally con- 

 ceded. Aside from the mere enumeration of the eleutherozoic forms, 

 a discussion of the structure of the animals is given in so far as this 

 may throw light on their genetic affinities. In some cases where the 

 observed facts seem to warrant the drawing of conclusions in regard 

 to the derivation and relationships of a certain type, this has been 

 done. The assumption of an eleutherozoic existence by certain 

 Pelmatozoa is of interest as bearing not only on the forms affected, 

 but also on the habits of the Pelmatozoa in general. For this reason 

 the influence of an eleutherozoic existence on the distribution and 

 segregation of the Pelmatozoa has been indicated though not treated 

 in any considerable detail. Finally, a general though brief discus- 

 sion of the maintenance of such a life among the Pelmatozoa in its 

 various aspects is given. 



It is here held the Echinodermata as we now know them are 

 descended primitively from an eleutherozoic stock. The eleuthero- 

 zoic archetype gave rise to a line of descendants among which a 

 sessile habit was gradually assumed. This form of life became 

 deeply ingrained m the fiber of the stock, and has left an indelible 

 impress on the structure of the organisms. When the term "eleuthe- 

 rozoic stock" is hereafter used the primitive organisms that had not 

 as yet acquired a sessile habit are meant. Similarly by ''statozoic 

 or sessile stock" we designate that body of echinoderms among 

 which a fixed mode of life was primarily assumed. It may well be 

 that absolute fixation did not obtain in all the phyletic lines of the 

 Echinodermata. In the case of the Holothurioidea particularly the 

 evidence is highly inconclusive. 



A tendency to break away from the sessile echinoderm stock and to 

 reassume a free mode of life is to be observed from practically the 

 earliest times to the present. Springing from the early, less spe- 

 cialized statozoic echinoderms, certain of these aberrant forms estab- 

 lislied lines that have been perpetuated as the great classes of the 

 Eleutherozoa. It is not within the scope of the present paper to 

 deal with these large groups except in a very general way. In later 

 times, as offshoots from the more highly specialized Pelmatozoa, the 



