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.
The mandibular symphysis extends backwards at least to the fifteenth tooth.
(a) The fourth mandibular tooth fits into a notch in the upper jaw.
(b) Fourth mandibular tooth fitting into a pit in the upper jaw.
The carapace, formerly referred only to the antennar-mandibular segments, may perhaps in fact contain elements from any number of other segments of head and trunk, Huxley, Alcock, Bouvier giving support to this opinion by the sutural or other divisional lines in Potamobius, Nephrops, Thalassina, and various fossil genera.
These may be formed by the modification of almost any of the appendages, often the antennules or antennae or some of the thoracic limbs, or even the mandibular palps (some Ostracoda).
The Ostracoda might have been derived from the same stock were it not that they retain the mandibular palp which all the Phyllopods have lost.
Probably it is fused with the D, Deuterocerebrum, consist protocerebrum, and may also be ing of ganglion cells be concerned in the history of the very longing to the second or peculiar paired eyes of Peripatus, mandibular somite.
The five pairs of appendages of the post-oral somites of the head or prosoma thus constituted all primitively carry gnathobasic projections on their coxal joints, which act as hemignaths: in the more specialized forms the mandibular gnathobases cease to develop.
Besides the first post-oral or mandibular pair, at least two succeeding pairs of appendages are modified as jaws.
The two somites following the mandibular or first post-oral or buccal somite carry appendages modified as maxillae.
The mandibular parapodia may be supposed during the successive stages of this history to have had, from the first, well-developed rami (one or two) of a palp-like form, so that the change required when the mouth passed away from them would merely consist in the suppression of the gnathobase.
On the whole the facts seem to be against this supposition, though we need not suppose that the gnathobase was very large or the rami undeveloped in the buccal parapodia which were destined to lose their mandibular features and pass in front of the mouth.
The poison-bag lies on the side of the head between the eye and the mandibular joint and is held in position by strong ligaments which are attached to this joint and to the maxilla so that the act of opening the jaws and concomitant erection of the fangs automatically squeezes the poison out of the glands.
The mandibular symphysis does not reach beyond the eighth tooth.
C. cataphractus is the common crocodile of West Africa, easily recognised by the slender snout which resembles that of the gavial, but the mandibular symphysis does not reach beyond the eighth tooth.
In Crustacea the fourth or mandibular somite never has less than the two following somites associated with it by the adaptation of their appendages as jaws, and the ankylosis of their terga with that of the prosthomeres.
The presence, in some genera, of a second row of mandibular teeth seems to indicate the former existence of a splenial element, such as exists in Siren among the Caudata and apparently in the labyrinthodonts.
(d) Rarely only one, and usually at least two, of the somites following the mandibular somite carry appendages modified as jaws (with exceptions of a secondary origin).
Only one somite following the first post-oral or mandibular segment has its appendages modified as jaws.
The mandibular somite bears a pair of gnathobasic hemignaths without rami or palps, and is followed by two jaw-bearing somites (maxillary and labial).
The head shows two jaw-bearing somites besides the mandibular somite (V, VI, in fig.
Posterior mandibular process.