THE MONTICULIPOROIDS. 



349 



the walls bounding contiguous autopores or mesopores, and the free 

 ends of which usually project above the general surface as spines or 

 blunt tubercles (fig. 227). The " acanthopores," when present, are 

 only developed in the peripheral or " mature " region of the colony ; 

 and they are readily recognised in tangential sections (fig. 226, ac), as 

 round bodies which usually show a minute central tube surrounded 

 by a thick and dark-coloured wall of laminated calcareous tissue. 

 Waagen has expressed the opinion that the " acanthopores " are 

 only immature tubes, but this is conclusively shown to be erroneous 

 by the fact that, while immature tubes can be readily demonstrated 

 in all specimens, the " acanthopores " are strictly confined to par- 

 ticular species of Monticuliporoids, and are uniformly absent in others. 



m 



Fig. 226. — A few corallites of Fis- 

 tulipora eriensis, Rom. : a, Auto- 

 pores ; m, Mesopores ; ac, Acantho- 

 pores. Enlarged about forty times. 



Fig. 227. — a, Profile view of a portion of the 

 surface of Dekayia asfiera, showing acantho- 

 pores; b, The same, viewed from above. Enlarged 

 about forty times. (After Nicholson and Foord.) 



Moreover, they differ entirely in structure from the young tubes, 

 and unlike the corallites (whether young or old) they project above 

 the general surface of the colony in the form of spines. Again, 

 when they are limited in number, the acanthopores occupy definite 

 positions as regards the ordinary tubes of the colony ; and, finally, 

 in many forms (fig. 230, d) the acanthopores are so numerous as to 

 render the hypothesis that they are of the nature of young corallites 

 quite untenable. It may, then, be taken as certain that the acan- 

 thopores represent a special morphological element in the Monti- 

 culiporoid colony ; but it is difficult to give any satisfactory explan- 

 ation as to their true nature. They may be regarded, with 

 considerable probability, as of the nature of aborted zooids ; and 

 possibly their nearest analogue is to be found in the " avicularia " 

 of the recent Polyzoa. This suggestion is supported by the fact 

 that in some living Polyzoans (as in Retepord) the avicularia are 

 attached to thickened tubes immersed in the substance of the 

 skeleton, the appearance of which in thin sections is very similar to 

 that observed in sections of those Monticuliporoids which possess 

 acanthopores. 



