268 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
in a diglyphie specimen (Fig. 2), and here vh. general resemblance to 
the specimen with three siphonoglyphs (Fig. 6) is so striking that 
I have felt almost justified in interpreting this specimen as a triglyphic 
animal, at one pole of which the directives, with the loss of the siphono- 
glyph, had given place to a group of non-directives. 
In tne preceding account I have intentionally avoided, as far as pos- 
sible, the use of the terms dorsal and ventral as applied to the two poles 
of the actinian’s body. This has not been because of objections that 
might well have been raised against these terms in themselves, as 
Haddon (789, p. 300) has done, but because of the more fundamental 
question of whether dorsal and ventral can really be distinguished in an 
adult Metridium. These terms, as is well known, may be applied with 
perfect precision to the adults of forms like Edwardsia, where the longi- 
tudinal muscles bear very unlike relations to the two poles of the animal ; 
but in forms like the diglyphic type of Metridium (Fig. 1), where the 
muscles of the pairs of non-directives are similarly related to both poles, 
this means of distinguishing dorsal and ventral is lost. It has been 
suggested that even in cases of this kind dorsal and ventral may still be 
distinguished, either by the conditions of the siphonoglyphs, — the ven- 
tral being better developed than the dorsal (Faurot, '05, p. 62), — or 
by the condition of the subsidiary mesenteries, — the more dorsal pairs, 
because of their earlier development, remaining larger than the ventral 
ones (Carlgren, ’93, p. 100). Unfortunately, these criteria, even sup- 
posing them to be true, which is by no means certain, cannot be em- 
ployed on the diglyphie type of Metridium because of the similarity of 
its two poles. So far as the adult diglyphie Metridium is concerned, I 
am obliged to confess that I can find no satisfactory criteria for the 
determination of dorsal and ventral relations. 
With the monoglyphie type the case seems simpler. It is generally 
stated that, when only one siphonoglyph is present, it is the ventral one ; 
but, as Carlgren (93, p. 100) remarks, so far as Sagartia is concerned, 
this statement has never been accompanied with any direct proof; nor, 
I may also add, has it been proved for Metridium. The argument used 
by MeMurrich (91, p. 133) to show that the single siphonoglyph in the 
monoglyphie Metridium is the ventral one may be used with equal accu- 
racy to show that this siphonoglyph is the dorsal one, for the argument 
advanced rests upon the sequence of the development of the mesen- 
teries, which, being unknown in Metridium, has simply been assumed by 
McMurrich. The case of Metridium seems to be precisely like that of 
