The serration of the surface here is of so marked a character that Langerhans considered this part of the velar folds to act as a masticating organ, grinding and rasping the food and mud which passed through the narrow slit.
What, then, are thesevelar folds, and how is it that the tubular muscles of these two segments become the velar muscles?
If this is so, then this homology ought to throw light on the extraordinary innervation of these tubular muscles by the branchialis profundus branch of the facial nerve and the velar branch of the trigeminal.
It is the only non-tubular muscle belonging to the velum, and by its contraction it draws the anterior portions of the velar folds apart from each other, and so opens the slit between them, through which the food and mud must pass.
The velar or mandibular nerve supplies in Ammocoetes the muscles of the lower lip.
In the centre of the velar area there is usually present a single long flagellum (fig.
The one limb is directed towards the velar area, the other towards the foot.
The tentacles are placed not within, but behind the velar area.
The permanent shell arises somewhat later as a series of median and lateral calcareous plates, first of all on the posterior part of the velar area, and subsequently on the mantle plates behind.
The rings finally unite, and form a single ring on a projecting velar ridge.
The velar area is in any case the only representative of the head.
In some cases amongst the Nudibranchiata (Haddon) and Pteropoda there are one or two long cilia in the middle of the velar area.
Rabl has shewn by sections that in Planorbis there are two lateral thickenings of the epiblast in the velar area; from which the supra-oesophageal ganglia become subsequently separated off.
By the time that these changes have been effected in the velum, the post-velar part of the embryo has become by far the largest section of the embryo, so that the velum forms a projecting disc at the front end of an elongated body.
The supra-oesophageal ganglia appear always to develop within the region of the velar area.
In phonology there are also very striking differences, apart from the treatment of the labialized velar qv already mentioned.
He finds that the former ganglia arise as an unpaired thickening of the epiblast in the centre of the velar area, and the latter as an unpaired thickening of the epiblast of the ventral side of the body between the mouth and the anus.
This eye originates from the ectoderm, within the velar area, and close to the supraoesophageal ganglia, usually at the base of the tentacles.
The ciliated band of the left side of the velar area is indicated by a line extending from v to v; the foot f is seen between the pharynx ph and the pedicle of invagination pi.
The foot now protrudes below the mouth, and the post-oral hemisphere of the trochosphere grows more rapidly then the anterior or velar area.
The outline of the velar area becomes strongly emarginated and can be traced through the more mature embryos to the cephalic lobes or labial processes of the adult Limnaeus (fig.
Farther up, within the velar area, the rudiments of the cerebral nerve-ganglion ng are seen separating from the ectoderm.
Three views of Limnaeus stagnalis, in order to show the persistence of the larval velar area v, as the circum-oral lobes of the adult.
The labio-velar series present a great variety of forms in the different Greek dialects, and in the same dialect before different sounds.
The l was probably first retracted, to differentiate it from the following dental; and then this velar l was opened into u.
The breaking was probably due to a premature lifting of the tongue under the influence of a following high vowel or a palatal or velar consonant, or to a premature partial closure of the lips in anticipation of a following labial.