PARKER: METRIDIUM 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 with 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 monoglyphic type, 
and that by two the diglyphie 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. I and 2) figured and described 
specimens of Metridium dianthus either with one or with two siphono- 
glyphs. The monoglyphic condition was also recognized for this species 
by Gosse (760, 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. 64). 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 (793, p. 104). Furthermore, the 
Dixons and Carlgren agree in stating that, though two siphonoglyphs 
may be present 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 (788, p. 120) ob- 
served that in Sagartia venusta, S. nivea, and S. mineata, either one or 
two siphonoglyphs may be present. 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-three 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 struo- 
tural type. The importance of these peculiarities from a systematic 
standpoint has already been appreciated, and in the more recent defini- 
tions of the TTexactinia the statement is made that these actinians possess 
