Part II. — Stickodacti/lince and Zoanthece. 191 



and arranged at a different height in different cells in such a way that in sections 

 they give rise to a very characteristic deeply-staining zone. Elsewhere in the 

 polyp the association of histological elements is more varied, glandular cells or 

 nematocyst-bearing cells mingling with supporting cells. 



In two of the present species of Parazoanthns — P. separatus and P. monostichus 

 — little or no reflected ectoderm is developed, and the mesenterial filaments are 

 simple throughout, that is, only the middle lobe is present, not the lateral lobes. 

 In longitudinal sections the ectoderm of the stomodseum is seen to be in 

 continuity with the similarly deeply-staining tissue along the free edge of the 

 mesenteries, but this is not continued for any distance up the faces of the latter ; 

 while transverse sections through the free edge of the mesenteries never present 

 any structure which can be regarded as the Flimmerstreifen. P. tunicans exhibits 

 on some of the mesenteries a weakly developed reflected ectoderm, and the 

 filaments are trilobed for a very short distance below the termination of the 

 stornodseum (PI. xv., fig. 4). 



In the figure which Haddon and Shackleton (1891, pi. lx., fig. 6) give of a 

 transverse section through the terminal region of the stornodseum of P. axinellce, 

 the reflected ectoderm is strongly displayed, and on the free mesentery the fila- 

 ment exhibits the characteristic trifoliate appearance. In the genus Parazoanthus 

 then every stage can be obtained in the presence or absence of the typical trifid 

 Actinian filament, the variation evidently being dependent in some degree upon 

 the dimensions obtained by the polyps. 



The absence of the Flimmerstreifen from the mesenterial filaments is now 

 known for several Actiniaria outside the Zoanthese, and is the condition exhibited 

 throughout the Madreporaria, as far as these have been studied. The character 

 must be regarded as indicative of a lower degree of Actinozoan development, and 

 in the two species of Parazoanthus mentioned, may be correlated with the very 

 diminutive size of the polyps not necessitating the same vigorous internal circu- 

 lation. 



Professor Haddon and Miss Shackleton draw attention to the fact that the 

 endoderm is often implicated in the upward reflection of the lower edge of the 

 stomodseurn. It is very noticeable in Parazoanthus axinellce, the appearance in 

 which species they figure. The same condition is also to be observed in all the 

 species of Parazoanthus coming under my notice, as well as in many other Actini- 

 aria and Madreporaria. In longitudinal sections it is evidenced by a strongly 

 mai-ked concave border to the mesentery as it leaves the stornodseum. 



As the authors referred to remark, it has probably no morphological significance, 

 and is no doubt exaggerated in a retracted state of the polyps. 



The members of the genus exhibit a certain relationship in regard to the 

 presence or absence of pigment granules and of zooxanthellse. It is usually 



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