They are at the same time both optic nerve-end cells, that is to say, retina cells, and corneagen cells or secretors of the chitinous lens-like cornea.
The almost spherical head is covered by a hood which can be retracted; it bears upon its side a number of sickle-shaped, chitinous hooks and one or more short rows of low 89 spines - both of these features are used in characterizing the various species.
A 2 to a 5, Posterior borders of the chitinous bases of the coxae of the second, third, fourth and fifth prosomatic limbs.
This telson may enlarge, it may possibly even become internally and sternally developed as partially separate somites, and the tergum may remain without trace of somite formation, or, as appears to be the case in Limulus, the telson gives rise to a few well-marked somites (mesosoma and two others) and then enlarges without further trace of segmentation, whilst the chitinous integument which develops in increasing thickness on the terga as growth advances welds together the unsegmented telson and the somites in front of it, which were previ ously marked by separate tergal thickenings.
In a few Entomostraca (some Phyllopoda and Ostracoda) the chitinous lining of the fore-gut develops spines and hairs which help to triturate and strain the food, and among the Ostracods there is occasionally (Bairdia) a more elaborate armature of toothed plates moved by muscles.
In many calcareous forms, .both Cheilostomes and Cyclostomes, the zoarium is rendered flexible by the interposition of chitinous joints at intervals.
H, Chitinous lining of the plate.
Nevertheless the filament is a complete tube formed of chitinous substance and clothed externally by ciliated epithelium, internally by endothelium and lacunar tissue - a form of connective tissue - as shown in fig.
It consists of a single layer of cells, continuous with those which secrete the general chitinous covering of the prosoma.
Bd, Enlargement of the oesophagus, armed with chitinous teeth..