TWIN GASTRUL/E AND BIPINNARLE. 581 



very close together, the infolding process may amalgamate them, giving 

 rise to an archenteron bifid in front and single behind (Fig. 5, gastrula ; 

 Fig. 16, bipinnaria). Again, in the case of a markedly bi-lobed blastula 

 an originally single invagination may, during inward growth, divide 

 into two branches (Fig. 6, gastrula ; Fig. 17, bipinnaria), but we must 

 often leave the question open whether there has been anterior fission or 

 posterior fusion of archentera (Figs. 7, 8, gastrula?). It is evident, 

 further, that anterior fusion of the archentera can take place (Figs 9-11, 

 gastrulse ; Fig. 19, bipinnaria). In Fig. 20 fusion of the expanded 

 stomachal regions of the archentera is exhibited by a specimen with 

 " back-to-back "' union. Fig. 18, on the other hand, illustrates a case of 

 " face-to-face '"' union in which the derivatives of two entirely separate 

 archentera share a common buccal cavity. In Fig. 21 (Mesodidymus) 

 the buccal cavity and rectum are single, while the oesophagus and 

 stomach are doubled and there is a composite enterocoelic cavity between 

 them. Fig. 7 shows triplicity in a modified form — the only instance of 

 triplicity observed. 



An examination of the various abnormal bipinnarias figured will show 

 that each archenteron tends to produce a pair of enterocoeles. In most 

 cases all four persist (Figs. 13-15, 17, 18, 20). Sometimes two from 

 different pairs (right of left pair and left of right pair) are united together 

 (Figs. 19, 21). More rarely these two have either never been formed or 

 have disappeared at a very early stage (Fig. 16). 



As regards the ciliated bands we note that their preoral and 

 postoral portions never become mixed — that is to say, the preoral 

 portion of one " twin " always unites with the preoral of the other, 

 and the postoral with the postoral. The general arrangement of 

 these bands, and of the larval fields they enclose, makes it patent that 

 quite remarkable powers of developmental " regulation " or " making 

 the best of things " must in many cases have been at work. 



SUMMARY. 



The various types of twin Luidia larvae may be classified according 

 to the same system as Double Monstrosities among vertebrates, the 

 alimentary canal of the larvae being taken as their representative axial 

 structure. 



The causation depends on early partial separation of cells or of cell 

 masses, accompanied by a minimal interference with the vitality of the 

 whole. 



