28 
tion of new ones; 3rd. Those in which there is @ combination of the definite and 
indefinite, the former on the dorsal side of the animal, the latter on the ventral. 
The plates which compose the outside or exoskeleton of cystidewz, are so arranged 
as not to close the orifices essential to the animal economy ; at all other points 
they join completely, forming an exact covering for the protection of the body, 
and the maintenance of those parts performing the various functions of these 
peculiar animals. The surfaces of the plates are usually ornamented with 
grooves, stria and rugosities, and occasionally exhibit pores, scattered singly or 
in pairs, which are supposed to serve as media for communication between the 
interior and exterior of the animal. The precise nature and function of these 
pores has not yet been perfectly ascertained. According to Forbes, there are 
only three pectinated rhombs found on the body of each cystidean, and their 
homologues have not been discovered in any section of the entire class of 
echinoderms. It is considered that the pectinated rhombs have situated on them 
ciliary organs, such as those found on the epaulettes of the larvee of echini. The 
porcs are usually disposed in rhomboidal figures, one-half in each contiguous 
plate, and every two pores of these rhombs are invariably united by small 
canals or grooves. These rhombs are considered so important that Professor 
Muller, of Berlin, has adopted them as a basis for classification. The mouth 
of this animal is an opening of moderate size placed on one side of the body, 
about its centre, but usually inclines towards either extremity. In some species 
it opened and shut by a sort of valvular apparatus, whereas, in others this 
peculiar arrangement is wanting. In cystideans provided with arms, the mouth 
is usually circular, and on its sides these appendages arise. In those species 
wanting arms, it is supposed that the mouth possessed a peculiar compensating 
power, by which it could protrude and retract its lips, and thus seize the material 
requisite for its digestive functions from the surrounding media. 
The ambulacral orifice. —This constitutes one of the most important open- 
ings in the structure of this species. It is placed near the centre of the 
upper part of the body, and usually between the bases of the arms, when 
present. These peculiar openings were a subject of considerable discus- 
sion amongst naturalists, and now there is little doubt that those vessels 
which constitute the ambulacral system communicate from the exterior to 
the interior, and vice versa, through these openings, as in the madreporic 
canals and tubercles of asterids and echinids, such as defined by Huxley. 
Although no specimen containing the soft parts has ever been discovered, by 
which we might be enabled to demonstrate the communication more pointedly, 
still, the natural inference is, that an animal such as a cystidean, stationary in its 
character, and possessing as its chief part a large body, to be sustained and 
nourished, would have ample circulating media throughout, to receive from the 
thoracic centre that degree of nourishment requisite to sustain the various 
functions of this strange organism in perfect activity. The third small opening 
(anus), is very minute, ancl frequently not observed in certain genera in which 
the wear and tear of the system is supposed to escape through the mouth, as in 
some of the star-fishes 
The Arms and Pinnule.—For some time the arms of cystidee formed the 
subjeet of a sharp controversy between Von Buch and Volbroth; the former 
denying the existence of these parts, while the latter adhered to the contrary 
idea. At present no one doubts the presence of arms in some shape or other. 
‘The Cystidean arms are considered to be like those of crinoids in many respects, 
although not remarkable for extensive development. For simplicity of descrip- 
tion they are divided into three classes : 1st, Cystidew, in which the body of the arm 
was not developed. but only the grooves and pinnule. 2nd. Cystidew, in which 
the arms were developed, but bent backwards and attached t» the body. 3rd. 
Cystidee with free arms. ‘This arrangement of arms must not be considered as 
an attempt at classification, for, on a closer examination of the genera, we find 
