PARKEE: METEIDIUM MARGINATUM. 261 



is obvious that this condition may be set aside as distinctly exceptional, 

 and, further, since the other specimens were almost equally divided be- 

 tween those with one and those wdth two siphonoglyphs, these conditions 

 may fairly be considered typical. It will be convenient in the subse- 

 quent discussion to designate these two types by special names, and I 

 shall call that characterized by one siphonoglyph the vionoglyphic type, 

 and that by two the diglyphic type. 



Variations in the number of siphonoglyphs have already been recorded 

 in other actinians. Thus, besides the observations of McMurrich already 

 alluded to, Thorell ('59, Tab. I. Figs. 1 and 2) figured and described 

 specimens of Metridhim dianilms either with one or with two siphono- 

 glyphs. The monoglyphic condition was also recognized for this species 

 by Gosse ('60, p. 12), who, in ignorance of Thorell's observations, sup- 

 posed this condition to be characteristic of the species, a mistake after- 

 wards corrected by Foot ('63, p, 61). The presence in some specimens 

 of one, and in others of two siphonoglyphs in M. dianthus, as first as- 

 serted by Thorell, has recently been confirmed by G. Y. and A. F. 

 Dixon ('91, p. 19), and by Carlgren ('93, p. 104). Furthermore, the 

 Dixons and Carlgren agree in stating that, though two siphonoglyphs 

 may be pi'esent in this species, one is the rule. G. Y. and A. F. Dixon 

 ('91, p. 20), moreover, have recorded one specimen of M. dianthus with 

 three siphonoglyphs. 



Representatives of the genus Sagartia also show variations in the 

 number of their siphonoglyphs ; thus G. Y. Dixon ('88, p. 120) ob- 

 served that in Sagartia venusta, S. nivea, and S. mineafa, either one or 

 two siphonoglyphs may be pi-esent. The same is probably true of 

 S. rosea (cf. F. Dixon, '88, p. 139), and of S. lactea (cf. McMurrich, 

 '94, p. 177). In specimens of Bunodes thallia, studied by G. Y. and 



A. F. Dixon ('89, p. 318), one, two, three, and even four siphonoglyphs 

 were observed, although in each of twenty-tliree adult specimens of 



B. verrucosa the same authors ('89, p. 322) found regularly two siphono- 

 glyphs. Finally Blochmann and Hilger ('88, p. 391) described a specimen 

 of Gonactinia in which traces of a third siphonoglyph seem to have 

 been present. 



It is evident from the foregoing account that in several actinians besides 

 Metridium a variation in the number of siphonoglyphs is not unusual, 

 though this variation may not be so pronounced as to constitute a struc- 

 tural type. The importance of these peculiarities from a systematic 

 standpoint has already been appreciated, and in the more recent defini- 

 tions of the Hexactiniathe statement is made that these actinians possess 



