)88 JAMES F. GEMMILL. 



PLATE III. 



{For lettering see p. 584. ) 



Fig. 16. — A bipinnaria in which there is doublmg of the mouth, buccal cavity, and 

 first portion of oesophagus. The anterior end of the archenteron at the end of gastrulation 

 must have exhibited a very sHght degree of doublmg. The frontal extremity of the larva 

 is broadened, but in other respects the bipinnaria looks normal ; cf. Fig. 5. 



Fig. 17. — A bipinnaria with anterior doubling and jjosterior union. The two frontal 

 fields and buccal cavities face one another and lead into a single oesophagus which is 

 contmued backwards into a smgle stomach, mtestine, and rectum, the anus bemg in the 

 middle of the posterior surface. There are two pairs of enterocoeles, the left enterocoele 

 of the right twin and the right enterocoele of the left twin being shown m the drawing 

 and the first-named has a hydroporic opening. The two preoral ciliated bands are 

 distinct from one another, but the postoral bands are continuous on opposite sides. The 

 circumoral food-collecting areas are also united. 



Fig. 18. — A double monster bipinnaria showing symmetrical ventral union of the 

 twin components, the fusion being somewhat greater at the anterior than at the posterior 

 end of the comi^osite larva. There are two frontal areas bounded by preoral ciliated 

 bands on opposite sides of the bipinnaria, but each frontal area and ciliated band is to be 

 looked upon as composite, that is, derived in part from one and in part from the other of 

 the twin components. The buccal cavity is single, but also composite, and it communi- 

 cates with the surface by two mouth openings on ojiposite sides of the larva. The two 

 postoral ciliated bands are ununited though they approach one another posteriorly. 

 (Esophagus, stomach, etc. are separate and there are two pairs of enterocoelic pouches. 

 The circumoral food-collecting areas merge into one another. 



Fig. 19. — A bipinnaria showing posterior doubling of the principal internal structures. 

 The view is from the dorsal aspect. The preoral ciliated band, the buccal cavity, and the 

 first part of the oesophagus are single, while the rest of the oesophagus, the stomach, and 

 the rectum are doubled. Note as regards the enterocoeles that the left one of the right 

 twin and the right one of the left twin have fused together to form a single composite 

 sac provided with a hydropore. The circumoral fields run freely into one another on the 

 ventral aspect of the larva. 



Fig. 20. — A double monster bipinnaria with the components united back-to-back by 

 their dorsal body-walls. Internally there is a composite stomach, but otherwise the 

 food-canals are separate. The rectum and anus of the twin to the left of the drawing 

 were lost. The two preoral and postoral bands and the two circumoral areas have 

 remained separate on either side, and there are two pairs of enterocoelic pouches. As 

 reo'ards internal structure we may describe this specimen as anakatadidymous, that is, 

 showing duplicity both anteriorly and posteriorly. 



Fig. 21. — A double monster bipinnaria in which although the buccal cavity and the 

 rectum are single, there is doubling of the intervenmg regions, namely, the oesophagus and 

 stomach. The view is from the dorsal aspect, and the duplex structures lie side by side, 

 and look in the same direction. Three enterocoelic sacs are present. The middle one 

 possesses a hydropore and has evidently arisen by the fusion of a right sac belonging to a 

 left twin with a left sac belonging to the right twin. Compare with Fig. 19. In other 

 respects the bipinnaria, though slightly broader than usual in its middle region, is super- 

 ficially almost normal. As far as internal structure is concerned we may describe this 

 bipinnaria as exhibiting the niesodidj'mous condition. 



