366 ox THE ANATOMY OF A SUPPOSED NEW SPECIES 



by a broken line. The outline of a primary septum (.S'. /) is also shown, fusing with the 

 columella below. The retractor muscles arise from the whole of the mouth-disc, but are 

 only attached to the lower two-thirds of the theca. The free edge of the mesentery is 

 everywhere covered by a mesenterial filament, which is directly continuous with the stomodoeal 

 epithelium above. 



tertiary mesenteries both above and below their open mouths (Fig. i. c and Fig. ii.). 

 Shorter pockets are found too, especially in sections where the open mouth is cut 

 through, projecting into the exocoeles outside the tertiary entocoele. In fact the 

 tertiar}' mesenteries would appear to continue for some distance into the extended 

 tertiary tentacles. 



The introversion of the secondary and primary tentacles gives rise to deep some- 

 what irregular invaginations — extending only below their open mouths — in the entocoeles 

 over their corresponding septa (Fig. I. D, and Fig. II.). The iuvaginated portions are 

 crescent-shaped in transverse sections owing to the projecting septa, and have no 

 side diverticula into the exocoeles. The mesenteries are attached, one at each side 

 of the bases of these tentacles, and cannot extend into them in any way, when they 

 are expanded. The tentacles appear as if they decrease considerably in length from 

 the tertiaries to the primaries. 



All the tentacles are covered with round, knobbed batteries of nematocysts, which 

 gradually decrease in size from their tips. At their bases these batteries are very 

 small, and pass almost imperceptibly — especially on the tertiary tentacles — into the 

 general ectoderm. 



Retractor muscles. The great longitudinal retractor muscles of the mesenteries 

 have the regular arrangement, typical of the Hexactiniae, i.e. on the sides facing one 

 another on all the pairs except the directives. Their course may be best seen by 

 reference to Figs. i. and II. The fibres of the tertiary mesenteries arise from the 

 tentacles alone, while those of the primaries and secondaries arise from the whole 

 of the mouth-disc. They then run on all the mesenteries diagonally across to end 

 near their attachments to the corallum. The lower part of all the mesenteries is free 

 from muscular fibres. 



On the faces of the mesenteries, opposite to the great i-etractor muscles, there are 

 a few isolated longitudinal muscles with a similar course. They are rather more 

 numerous on the directive mesenteries, the great retractor bands of which are not as 

 large as those of the other primary mesenteries. There do not appear to be any 

 definite protractor or transverse muscles. The circular sphincter muscle — Rotteken's 

 muscle — is a broad, diffuse band situated immediately below and outside — morphologi- 

 cally speaking — the tertiary tentacles (Fig. ii.). 



Stomodoeum. The stomodoeum is about 1 mm. in length in the contracted polyp. 

 Its mouth is an elongate or oval slit, lying in the long axis of the directive septa, 

 without any trace of gonidial grooves at its ends. Below the mouth its lumen is 

 very irregular, being often drawn out into deep pockets between the septa. 



Mesenteries. The tertiary mesenteries are attached to the mouth-disc in the 

 tentacles, but do not reach to the aboral or basal body wall. Their free edges, except 



