382 



INVEETEBRATA 



CHAP. 



Unfortunately Masterman's fascinating hypothesis has not been 

 sustained by subsequent workers. Thus, Goodrich shows clearly that 

 the so-called prae-oral coelom is merely a portion of the blastocoele, 

 or primary body-cavity, corresponding to the cavity surrounding the 

 gut in a Trochophore larva ; and that the nephridia cannot be homo- 

 logous with collar pores, because they end blindly internally and are 

 beset with solenocytes projecting into the blastocoele ; they are in 

 fact archinephridia like those of Annelida. Goodrich admits the 

 existence of collar and trunk coelomic cavities, but Ikeda, de Selys 



Longchamps, and Shearer 

 ap deny that these arise as endo- 



dermic diverticula. 



In justice to Masterman 

 it ouglit to be noted that 



arch 



Fig. 309. — Diagramm.itic frontal section of tlie 

 Actinotrocha larva of Phoronis [sp^) captured 

 near Ceylon, (After Goodrich. ) 



ap, apical plate ; colx, collar coelom ; int, intestine ; 

 mtr, tentacles of the metatroch ; nepk, nephridium ; 

 oes, oesophagus ; pr.hs, prae-oral blood space ; s.n.p, 

 subneural pit ; sol, solenocytes of the nephridia ; fit, 

 stomach ; tr.c, trunk coelom ; t.tr, telotroch. 



mes 



Flu. 310. — Longitudinal hori- 

 zontal section of the embryo 

 of an Australian species of 

 Phoronis. (After Caldwell.) 



arch, arclienteron ; mch, mesen- 

 chyme cells ; itics, diverticula of the 

 archenteron giving rise to the meso- 

 derm of the trunk-cavities according 

 to Caldwell. According to Shearer, 

 an ectodermic pit giving rise to the 

 tnbe;{ of the two nephridia. 



Caldwell described the coelom as arising from two posterior outgrowths 

 from the gut (Fig. 310) ; and though Shearer asserts that the bilobed 

 ingrowth seen by Caldwell is an ectodermic pocket which gives rise 

 to the tubes of the nephridia, yet the fact that the first trace of the 

 coelom seen by him was an unpaired bilobed sac, lying close to the 

 dorsal surface of the hinder end of the gut, renders it possible that after 

 an Caldwell and Masterman are right and that the trunk coelom does 

 arise as a pair of posterior diverticula of the gut, at least in some species. 

 Further work is needed to obtain complete certainty on this point. 



