DIVERGENCIES IN THE METAMORPHOSIS, ETC. 47 



adductor muscle (Fig. 16, stri), formed from elongate mesoderm-cells 

 which become arranged side by side and attached to the two valves 

 of the shell. This adductor in Ostrea lies dorsally to the alimentary 

 canal, and thus corresponds in position to the anterior adductor of 

 the Dimyarian forms, which also lies dorsally to the oesophagus (Fig. 

 31, vsm, p. 7- r >). The adductor in the adult Monomyarian, how- 

 ever, lies ventrallv to the intestine (Fig. 31, hsm) ; it thus occupies 

 the same position as the posterior muscle in the Dimyaria, and is 

 undoubtedly to be homologised with this latter. The adductor of 

 the larval oyster therefore cannot he the same as that of the adult. 

 This difference, which has been emphasised by several investigators 

 (Huxley, Hokst, etc.), is explained by the study of the later 

 development (Jackson, Nos. 22 and 23.) 



A larval stage in which only one adductor muscle (the anterior) is- 

 present, or in which the anterior adductor is better developed than the 

 posterior adductor, which is only in the act of appearing, is met with in a 

 large number of Lamellibranchs, e.g., < 'ardium, Montacuta, Mortlolaria, 

 Mytilus, Dreissensia, Pisidium. In the Unionidae also the anterior 

 adductor seems to appear first, as, indeed, is the case in nearly all 

 the Lamellibranchs as yet investigated in this respect. 



In Cyclas, on the contrary, according to Ziegler, the posterior adductor 

 develops before the anterior, but it has already been pointed out that in the 

 related form, Pisidium, the anterior appears first. Lacaze-Duthiers (No. 28) 

 maintained that the posterior adductor develops first in Mytilus, but this 

 is due to the fact that the larval stages examined by this author as well as by 

 Loven (No. 33) were too old. According to Wilson (No. 59), in the young 

 Mytilus larva the anterior adductor develops earlier than the posterior, and 

 this is also the case in the nearly related form Dreissensia (Korschklt, 

 No. 11). 



After the anterior adductor has appeared in Ostrea, a posterior 

 adductor lying ventrally to the intestine arises in the same manner 

 as in the Lamellibranchs mentioned above (Jackson). Ostrea, and 

 no doubt the other Monomyaria as well, possess for a time two 

 adductors (an anterior and a posterior) of almost equal strength, and 

 thus resemble the Dimyaria. Only as the anterior of these two 

 muscles degenerates does Ostrea assume a Monomyarian condition. 



Even if the Lamellibranch larvae do for a time possess only one ad- 

 ductor, we have no right to speak, as has often been done, of a Monomyarian 

 stage, and to consider the permanent condition of the Monomyaria as having 

 arisen through an arrest of development in this direction, i.e., through the 

 defective development of a second muscle. The Dimyaria hence do not pass 

 through a Monomyarian stage in the proper sense of the term, but the 



