- RR in contact all round; first brachial usually quadrangular.
The brachial plexus is formed by four or five of the lowest cervical nerves; the last nerve of this plexus often marks the boundary of the cervical and thoracic vertebrae.
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.
(A portion of the fringe of cirri is removed to show the brachial membrane and a portion of the spiral extremities of the arms.) FIG.
D, h, Brachial appendages.
May be, as in Cistella, applied flat to the inner surface of the dorsal mantle fold, but more usually they are raised free from the body like a pair of moustaches, and as they are usually far too long to lie straight in the mantle cavity, they are folded or coiled up. The brachial skeleton which in many cases supports the arms has been mentioned above.
The development of the brachial supports has been studied by Friele, Fischer and Oehlert.
Left brachial artery arising from a common innominate trunk, instead of coming off separately from the aortic arch.
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.
In this development of brachial extensions of the theca the genital organs were involved, and their ripe products formed at the ends of the brachia or in the branches therefrom.
An aborally-placed motor nerve-centre gives off branches to the stroma connecting the various plates of the theca and of its brachial, anal and columnar extensions, and thus co-ordinates the movements of the whole skeleton.
- RR separated by a heptagonal anal; first brachial usually quadrangular.
- RR separated by a hexagonal anal; first brachial usually hexagonal.