HEXACTlNI^ FROM ISTBW SOtJTH WALES. 



4^ 



the oesophagus throughout their length (PI. 4. fig. 3, and text-figure A). 

 The pairs 5 5 and 6 6, probahly the last two pairs o£ mesenteries to be 

 formed (15), are not attached to the lower half of the oesophagus in the adult 

 form. Owing to the capitular region having been cut longitudinally, I have 

 not been able to determine whether or not they are attached to the upper 

 region of the oesophagus. In the larval form the twelve perfect mesenteries 

 only are present. These are fully formed, bearing muscle-pads, digestive 

 endoderm, and mesenterial filaments, and the pairs 5 5 and 6 6 are free 

 throughout their entire course. 



Laferal exocoele 



Sulca-exoccele 



3 5 



Diagrammatic section through lower oesophageal region of Peachia hilli. 



The longitudinal retractor muscles are unilateral and exceedingly po^^erful. 

 Distinctly narrower peripherally they widen towards their internal margin 

 and end a short distance from the oesophagus in a large half renal curve 

 (PI. 4. figs. 3, 8). The long narrow mesogloeal processes on which the muscle- 

 cells are placed frequently branch, and the processes show a central dark- 

 staining core. A number of short thick muscular outgrowths occur on the 

 mesentery between the longitudinal retractor and the oesophagus (PI. 4. 

 fig. 8, mw.). 



The parietal muscle, bilateral and well-developed, is situated on short 

 blunt processes of the mesogloea (PI. 4. fig. 8, p./n.). These on one side 

 extend to a greater width and join the fibres of the longitudinal retractor, on 

 the other in the perfect mesenteries they often form, especially in the 

 oesophageal region, little tufts or brush-like outgrowths. The mesenteries 

 diminish in width towards the posterior end of the column and with them 

 the retractor muscles diminish also. At the posterior end of the scapus they 

 form two groups (PL 4. fig. 7). One consists of the pairs 1 1, 2 2, and 3 3, 

 which still retain some digestive endoderm. Of these, 3 3, formerly carriers 



