They secrete a cuticle which never approaches in thickness the often calcified cuticle of Arthropods.
Stomach generally provided with chitinous or calcified masticatory plates.
Cephalic shield pointed behind; shell internal, chiefly membranous, with calcified nucleus, nautiloid; parapodia forming fins.
In addition to these, there exists in the interior of the dorsal valve of some genera a variously modified, thin, calcified, ribbon-shaped skeleton for the support of the ciliated arms, and the form of this ribbon serves as one of the chief generic characters of both recent and extinct forms. This brachial skeleton is more developed in some genera than in others.
In the Rhynchonellidae there are two short slender curved laminae, while in many genera and even families, such as the Productidae, Strophomenidae, Lingulidae, Discinidae, &c., there exists no calcified support for the labial appendages.
The old idea of the circulating blood being supersaturated with lime salts which in some way had first become liberated from atrophying bones, and then deposited, to form calcified areas in different tissues will have to be given up, as there is no evidence that this " metastatic " calcification ever takes place.
- Pudic artery showing calcified areas which have taken into their protoin the muscular coat of the vessel.
Body-wall not calcified, body-cavity absent.
Body - wall membranous or calcified, body cavity distinct.
- Zooecia with more or less calcified walls.
The body-wall is extensively calcified in the Cyclostomata and in most Cheilostomata, which may form elegant network-like colonies, as in the unilaminar genus Retepora, or may consist of wavy anastomosing plates, as in the bilaminar Lepralia foliacea of the British coasts, specimens of which may have a diameter of many inches.
Anomia; byssus large and calcified; British.
On the outside of the shell is a non-calcified layer of conchiolin called the periostracum, secreted by the thickened edge of the mantle.
In one or two cases the epithelium of the foot secretes a calcified shell, which is either free as in Argonauta or adherent as in Hipponyx.
The cuticle, in some species very thick, contains numerous spicules which are long, hollow and calcified; they are secreted by epithelial papillae.
The last-named species has a straight symmetrical abdomen, with the penultimate segment expanded and strongly calcified to form a back-door to the very unconventional habitation.
These teeth are connected with a framework of movably articulated ossicles developed as thickened and calcified portions of the lining cuticle of the stomach and moved by special muscles in such a way as to bring the three teeth together in the middle line.
Such characters are found in any primitive, sedentary group. More peculiarly Echinoderm features, in which the Pelmatozoan nature is manifest, are the enclosing of the viscera in a calcified and plated theca, for protection against those enemies from which a fixed animal cannot flee; the development, at the aboral pole of this theca, of a motor nerve-centre giving off branches to the stroma connecting the various plates of the theca and of its brachial, anal, and columnar extensions, and thus coordinating the movements of the whole skeleton; the absence of suckers from the podia, which, when present, are respiratory, not locomotor, in function.
- Echinoderma with the viscera enclosed in a calcified and plated theca, of which the oral surface is uppermost, and which is usually attached, either temporarily or permanently, by the aboral surface.
- Echinoderma in which the theca, which may be but slightly or not at all calcified, is not attached by any portion of its surface, but is usually placed with the oral surface downwards or in the direction of forward locomotion.
Even in the whalebone whales their germs are formed in the same manner and at the same period of life as in other mammals, and even become partially calcified, although they never rise above the gums, and completely disappear before birth.
In others, which represent the perichordal type, the greater share of the formation of the whole vertebra falls to the (paired) dorsal cartilage, but there is in addition a narrow ventral or hypochordal cartilage which fuses with the dorsal or becomes connected with it by calcified tissue; the notochord is thus completely surrounded by a thick sheath in tadpoles with imperfectly developed limbs.
In the shell of Lamellibranchs three distinct layers can be distinguished: an external chitinous, non-calcified layer, the periostracum; a middle layer composed of calcareous prisms perpendicular to the surface, the prismatic layer; and an internal layer composed of laminae parallel to the surface, the nacreous layer.
Calcified specimens are especially characteristic of the British Carboniferous formation; their preservation is equally perfect with that of the silicified fossils, and their investigation by Witham, Binney, Williamson and others has proved no less fertile.