AND ON THE STRUCTURE OF ITS CILIA. 1] 
animals, and in the embryo condition of more elevated classes. They are the organs of 
motion and of respiration in the Polygastrica and Rotifera. They are the instruments 
of respiration, and produce the currents to the mouths of the polypi of Zoophytes. 
They are the organs of locomotion of the reproductive gemmules of Poriferous and 
Polypiferous animals, and probably produce the currents through the pores of the former 
class. They are important parts of the respiratory apparatus of adult Conchifera, and 
I have shown them to be the organs of locomotion in the embryos of naked and tes- 
taceous Gasteropods. They constitute the organs of locomotion and of respiration in 
several genera of Acalepha allied to the Beroé, of which M. Blainville has formed a 
family thence called Ciliogrades. 
These minute hair-like organs are variously disposed on the surface of animals ac- 
cording to the object of their motions, whether for respiration, progressive motion, or 
obtaining food. They move with great regularity and velocity, and they occur so nu- 
merous on a single animal, that I have calculated about four hundred millions of them 
ona single Flustra foliacea. The cilia are generally organs so minute, that with the 
aid of the microscope we can only discover their outward form, their position, and the 
direction of their motions, their intimate structure entirely escaping observation. They 
appear like flat tapering filaments prolonged from the homogeneous cellular tissue of the 
body to which they are attached. The magnitude, however, of these organs in the 
Beroé, and the transparency of the parts around them, enabled me to perceive that in 
this animal they are not single fibres, but consist of several straight short transparent 
filaments placed parallel to each other in a single row, and connected together by the 
skin of the animal, like the rays supporting the fins of a fish, These fins are of the 
same breadth with the bands to which they are attached, and they extend from the 
mouth to the anus, there being about forty on each band. Viewed with the aid of a 
lens, the parallel fibres appear like transparent tubes, sometimes a little detached from 
each other at their free extremities by injury done to the connecting membrane, and at 
these parts the isolated spines projected stiffly outwards. Where the fins were quite 
entire, the membrane connected the tubular rays to their extremity, where the fin pre- 
sented a slightly rounded outline. Dr. Fleming observed in Beroé ovatus water moving 
in vessels along the middle of the bands to which the cilia are attached ; and M. Audouin 
has observed that in the closely allied genus Idya, the water is sent into the cilia, which 
he considers as respiratory organs. The animals can change the direction of the cur- 
rents of water in the vessels, and also the direction of the motions of the cilia. When 
the cilia are in active vibration, the motion appears like the continued undulations of 
a fluid along the surface of each band. This structure cannot be observed in the mi- 
nuter forms of the cilia in other classes ; but from the similarity of their arrangement, 
and their mode of action, it is probable that the structure is similar. The cilia of Tri- 
choda patula, Mill., a minute animalcule, are disposed in longitudinal series from the 
mouth to the anus, precisely as in the Beroes. It does not appear probable that the 
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