HEXACTIKH^. FROM NEW SOUTH WALES. 49 



Mesenteries and Acontia (PL 5. fig. 13, mes.,a.). — There are tliree cycles 

 of mesenteries ; the formula for each sextant being PiilIiiP. The mesenteries 

 of the first cycle number six pairs, including two pairs of directives (PL 5. 

 fig. 13, me}'^. They alone are perfect and carry the gonads ; there are 

 also six pairs of imperfect mesenteries of the second (PL 5. fig. 13, me.^) 

 and twelve pairs of the third cycle (PL 5. fig. 13, 7ve.^), all of which are 

 in a very rudimentary state. There is no trace of a fourth incomplete cycle 

 present in several of the species already described, and the suggestion, 

 therefore, of Kweitnievvski (31) that the partial development of the fourth 

 cycle might be a generic character is incorrect. This fourth cycle is absent 

 also in JP. vermiformis. 



Perfect Mesenteries. — The longitudinal retractor muscle of the perfect 

 mesenteries is contained in a muscle-pad which is large and reniform in 

 cross-sections. The distal end and external appearance of this muscle-pad 

 is shown in fig. 10, m.p., PL 5. It continues proximally to within a short 

 distance of the pedal disc. The muscle-cells are situated on slight, long, very 

 numerous and richly-branched processes (PL 5. fig. 13, l.r.ni.'). Distally, 

 beyond the level of this muscle-pad, short blunt muscular processes occupy 

 the greater width of the perfect mesenteries. Here the mesogloea is thicker 

 and contains isolated cells. The perfect mesenteries are also supplied with 

 a strong parieto-basilar muscle on plaitings of the mesogloea. This is 

 produced on one side into a long brush-like tuft (PL 5. fio-. 13, pJun.). 

 The mesenterial filaments of the perfect mesenteries are divided into several 

 branches. A septa is apparently present in the perfect mesenteries but 

 owing to tears caused by removal of sand and to imperfect preservation this 

 point could not be decided. 



Incomplete Mesenteries (PL 5. fig. 13, me}, me}). — The incomplete mesen- 

 teries of the second and third cycles are rudimentary, without gonads 

 mesenterial filaments, or muscle-pads. They possess strong blunt muscle 

 processes slightly branched on side. The mesenteries of the second cycle 

 are only slightly wider than those of the third. In the region of the pedal 

 disc, where the perfect mesenteries are narrower and have lost their muscle- 

 pads, the internal borders of all three cycles become connected too-ether in 

 groups. The largest group consists of 16 mesenteries, whose inner border 

 forms part of a circle lined by muscle processes. Examples of somewhat 

 similar coalescences of several mesenteries of different cycles have already 

 been recorded by Dixon (12) and by Parker (19). 



The Acontia (PL 5. figs. 10, a., 13, a.) are very large and are visible 

 immediately on opening the coelenteron. They form very white coiled tubes 

 of considerable length, some of which pass well into the capitulum (PL 5. 

 fig. 10, a.). Transverse sections show that they are rounded and one side is 

 fully armed with nematocysts, but the histology is not well preserved. No 

 cinclides are visible. 



LINN. JOURN. ZOOLOGY, VOL. XXXll. 4 



