268 bulletin: museum of compakatiye zoology. 



in a diglyphic specimen (Fig. 2), and here the 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, wnth the loss of the siphono- 

 glyph, had given place to a group of non-directives. 



In the 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 ('89, 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 doi'sal 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, '95, 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- 

 ploved on the diglyphic type of Metridium because of the similarit}- of 

 its two poles. So far as the adult diglyphic 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 monoglyphic type the case seems simpler. It is generally 

 stated that, when only one siphonoglyph is present, it is the ventral one ; 

 but, as Carlgi-en ('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 pi'oved for ^letridium. The argument used 

 by McMurrich ('91, p. 133) to show that the single siphonoglyph in the 

 monoglyphic 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 



