26 PERCY SLADEN TRUST EXPEDITION 



them having developed their longitudinal muscles on adjacent faces" (p. 119). Which of 

 these two views is applicable to the forms included in Group II can be determined only 

 after making a comparative study of their development. 



Faurot (40, 42) applied the term " couple " to two mesenteries that appear at the 

 same tiiue, one on each side of the sagittal plane, and "pair" to two adjacent mesenteries, 

 which lie on the same side of the sagittal plane, facing each other. According to this 

 nomenclature the twelve primary mesenteries are formed in couples which appear arranged 

 in pairs in the adult, whereas the mesenteries of the succeeding orders are formed in pairs, 

 one pair in each exocoele. In 1900, Bourne (16), on the other hand, used these words in 

 exactly the opposite sense ; in a footnote on p. .39 he added that "it is convenient when 

 speaking of the adult arrangement of the mesenteries to use the word ' couple,' when of 

 their developmental sequence to use the word 'pair.'" But in 1905 {17)- he reverted to 

 Faurot's use of "pair." Bourne's original use of "pair and couple" is, however, to be 

 preferred when speaking of forms which are bilaterally symmetrical, and since the 

 Anthozoan polyp is said to have a primary bilateral symmetry, "pair" is to be used to 

 denote the mesenteries that appear on either side of the sagittal plane, and couple when 

 referring to their adult arrangement*. The mesoglaeal pleats of the mesenteries are either 

 simple or sub-divided and vary in their shape, size and radial distribution in the different 

 species. In the stomodseal region of the polyp the mesenterial mesoglsea is devoid of 

 entoccelic pleats for a variable distance from its stomodseal attachment. Below the 

 stomodseum, the entocoelic pleats usually become broader and cover nearly the whole of 

 the width of the mesentery. In a few species, e.g. Favia favus (Forsk.) (PL 4, fig. 36) 

 and Echinopora hirsutissima (Ed. and H.) (PL 2, fig. 20), short exoccBlic pleats are present 

 to a short distance from the stomodseal attachments of the primary mesenteries, on which 

 the muscle-strands are better developed ; this may perhaps be because the polyps in this 

 species have assumed an oblique position. In the polyps of Mycedium okeni Ed. and H. 

 that I have examined the exocoelic pleats are much broader and thicker than the entocoelic 

 ones and extend over the inner half or two-thirds of each principal mesentery, the latter 

 being as a rule restricted to its outer quarter. This peculiarity may be due to the 

 completely horizontal position which the polyps have assumed in that species. The 

 mesoglaea is not of uniform thickness along the entire width of the mesentery. 



Though the contraction of the polyps may affect the condition of the mesenterial 

 mesoglsea, I find its characters, viz., relative thickness, the shape, size and distribution of 

 the pleats (both entocoelic and exocoelic), retaining a specific constancy. The Hertwigs and 

 the Actinologists after them attached the same value to the mesoglseal pleats of the oral- 

 disc. The endoderm is usually vacuolated. The vacuoles when definite are either oval or 

 goblet-shaped (PL 1, fig. 4) and, as I have suggested, may be capable of ingesting and 

 digesting food-particles. The endoderm is not usually of even thickness along the entire 

 width of the mesentery. Its relative thickness varies in the different species. Behind 

 the attachment of the mesenterial filament, the endoderm is usually swollen, pad-like in 

 transverse section, and is constricted off from the filament ; its two wings were termed 



* H. V. Wilson (122) used " pair " with reference to both the developmental history and the adult arrange- 

 ment of the mesenteries. 



