The os articulare bears on its inner side the inner mandibular process which serves for the insertion of part of the digastric muscle or opener of the mouth; another portion of this muscle is attached to the os angulare, which frequently forms a FIG.
All of these are to be regarded as primitively post-oral, but in the course of development the mouth moves back to the mandibular segment, so that the first three somitesocular, antennal and intercalary - lie in front of it.
17), is the mouth or oral piece; the second, explained by the presence of a " latent endoderm-group " in those the antennal segment; the third, the intercalary or prae-mandibular invaginations.
The quadrato-mandibular joint is placed in a level far behind the occiput.
The piercing portions of the latter - representing their inner lobes or laciniaelie median to the mandibular piercers in the natural position of the organs.