120 GEORGE W. FIELD. 



described for tlie nephridia of certain worms, e. g. by Bergh 

 for Criodrilus. From the simple conditions in the formation 

 of the mesodermal elements above desccibed is justified the 

 belief that the condition here found is phylogenetically ante- 

 cedent to that of the Annelids. 



The function of the water-pore is difficult to determine. 

 That its function in the adult as the stone canal is excretory 

 is claimed by Hartog (19), but opposed by Cuenot and H. 

 Ludwig. Bury saw exhalent currents but never inhalent, 

 though, as he says, this does not prove that there are not inha- 

 lent currents. He found that the movements of the cilia are 

 in such a direction as to cause an exhalent current. From 

 Bury's observations, and by elimination, one is almost forced 

 to ascribe to it, at least in the larva, the excretory function, 

 though in the adult this may be obscured by other functions ; 

 and the pore canal and water-pore of the Echinoderm larva 

 seem in many ways to be comparable to the nephridia of Anne- 

 lids, and to be ancestral in character. 



The Echinoderm larva is a form which has developed along 

 the phylogenetic line, and is in many ways differentiated and 

 capable of free existence — an animal with a well-differen- 

 tiated digestive tract, and having locomotor apparatus, ente- 

 rocoels, excretory system, and well provided for respiration ; 

 to these have been coBnogenetically added transparency as a 

 protective adaptation, and the formation of long arms for 

 protection, but primarily as a means of increased locomotor 

 powers. The great length of the arms has probably been 

 acquired since the time when the metamorphosis began to be 

 accelerated by its earlier beginning ; i. e. originally metamor- 

 phosis did not begin until after the larva had become fixed to 

 some support, but secondarily the beginning has been pushed 

 forward, so that it now occurs long before fixation. The long 

 arm-like projections of the larva are to be explained through 

 the necessity for increased powers of locomotion on account 

 of the weight of the adult starfish developing in the hinder 

 end of the larva. But the greatest of the coenogenetic modi- 

 fications is that whereby the typical larva acquires the dif- 



