EARLY DEVELOPMENT OF A STARFISH 465 



tion are genetically related, but the interpretation of certain 

 details is not completely satisfactory. For example there is a 

 lack of agreement relating to the homologies of the coelomic 

 cavities. It will be remembered that Balanoglossus develops 

 five pouches, an anterior unpaired vesicle and two succeeding 

 pairs which become the proboscis, collar and trunk coelom re- 

 spectively. In Balanoglossus clavigerus, according to Heider 

 ('09) the anterior vesicle arises as a diverticulum from the distal 

 end of the archenteron, and after separating completely, forms 

 the future proboscis pore, before coming in contact with the 

 apical plate. After a time the vesicle withdraws from the plate, 

 though retaining its connection by means of a muscular strand, 

 while mesenchyme cells from the strand and undetermined neigh- 

 boring regions migrate to the oesophageal region where they 

 become transformed into circular muscles. 



The resemblance in the behavior of the anterior vesicle in Balano- 

 glossus clavigerus and Pateria mineata is decidedly striking. In 

 the starfish the vesicle arises in the form of mesenchyme cells 

 which subsequently unite; in Balanoglossus it originates as a 

 direct outgrowth of the archenteron. In Asterias rubens and a 

 few other species of starfishes Gemmill (14) has described the 

 rudiments of posterior enterocele pouches which either fuse with 

 the left hydrocele or break up into mesenchyme (and possibly. 

 unite later with the hydrocele, though this is not indicated). 

 In other words it is well known from this and numerous other 

 instances that there is no fundamental difference between a vesicle 

 formed as an outpouching of the gut and the precocious develop- 

 ment of mesenchyme with subsequent fusion. In Pateria mine- 

 ata the anterior vesicle soon disappears, the component mesen- 

 chyme cells wandering off into the blastocele, but before its dis- 

 appearance it has come into close contact with wandering cells 

 from the apical plate. No pore unites this anterior vesicle with 

 the exterior, but its behavior is such that I am convinced it is 

 the homologue of the proboscis coelom of the Enteropneusta. 



There is, so far as I know, no other starfish which develops a 

 distinct, independent anterior vesicle. In several species the 

 blind end of the archenteron becomes expanded, thin-walled and 



