No. i.] MORPHOLOGY OF THE ACTINOZOA. 131 



primary mesentery of which possessed longitudinal muscles on 

 both the dorsal and ventral face. 



But more important examples of a change from the normal 

 arrangement of the muscles are to be found among the Sagar- 

 tidae. G. F. Dixon (88) has shown that in three species of 

 Sagartia (S. venusta, S. nivea, and 5. miniata) a single siphono- 

 glyphe only is frequently present, and in 5. venusta he found 

 that a form with only one siphonoglyphe possessed only a single 

 pair of directives. F. Dixon ('88) confirmed this observation, 

 and showed that it is the rule with 6". nivea and .S. rosea also ; 

 and I have found the same state of things to occur in Metridium 

 marginatum. In this species, of which I have had an oppor- 

 tunity of examining a large number of individuals, the presence 

 of a single siphonoglyphe is almost, if not quite, as frequent as 

 the more normal arrangement. 



The arrangement of the mesenteries in one specimen exam- 

 ined is shown diagrammatically in Fig. 1, PL IX. It will be seen 

 that apparently only one siphonoglyphe is present, and with it 

 is connected a pair of directive mesenteries (III). At the other 

 extremity of the stomatodaeum is a pair of mesenteries (IV) cor- 

 responding to the directives of other forms, but having their 

 longitudinal muscles upon the contiguous faces, as is the rule 

 in the lateral pairs of mesenteries. Now, there can be no ques- 

 tion here as to the identity of the forms with only one pair of 

 directives and one siphonoglyphe with those with two, nor can 

 there be any doubt that the forms with two pairs of directives 

 are the more normal ones, the condition in which there is only 

 one pair being a secondary modification, since the former ar- 

 rangement is characteristic of the ancestral Halcampidae and 

 Edwardsiae. If we consider the siphonoglyphe which persists 

 to be in all cases the ventral one, we have a case in which there 

 is a secondary reversal of the arrangement of the musculature 

 of the fourth pair of primary mesenteries in nearly half the 

 individuals of a species. 



A further point of interest lies in the fact that this reversal 

 is associated with the absence of a siphonoglyphe. 



G. F. and A. Y. Dixon ('89) have described an interesting 

 abnormality in Bunodes tJiallia, one specimen of which pos- 

 sessed three pairs of directives in connection with tJiree siphono- 

 glyphes. Blochmann and Hilger also describe a variation in the 



