366 THE BIOLOGY OF MARINE ANIMALS 



coelenterates, ctenophores, turbellarians, nemertines, small annelids and 

 urochordates. In higher groups ciliated epithelia may occur in more 

 restricted tracts on the external surface as well as lining various internal 

 cavities and tubes. 



Cilia function only in an aqueous medium, either by beating against 

 the surrounding medium when they are situated externally, or by propelling 

 internal fluids. Despite their minute dimensions (rarely exceeding 15// in 

 length), by their conjoint efforts they perform a remarkable variety of 

 functions with great efficiency. In small organisms they are the sole or 

 principal means of locomotion. Other functions which they subserve, 

 described on other pages, include respiration, collection and transport of 

 food particles, movement of digestive fluids, circulation of body fluids, 

 removal of waste materials and propulsion of genital products. Among 

 sedentary invertebrates the activity of cilia in creating feeding currents is 

 particularly noteworthy (vide Chapter 5). The volumes of water pumped by 

 such mechanisms are indicated in Tables 4.5 and 4.6 (p. 169). 



Structure of Cilia 



With few exceptions the structure of cilia and flagella is remarkably 

 uniform throughout the animal kingdom. Cilia of living cells are optically 

 homogeneous. When examined with polarized light they show positive 

 form and intrinsic birefringence, indicating the presence of longitudinal 

 protein fibres. The filament arises from a small basal granule, which is 

 believed to be derived from the centrosome. In ciliated epithelial cells an 

 intracellular system of fibrils (rootlets) extends from the basal granules 

 into the distal cytoplasm towards the nucleus, and horizontal fibrils link 

 the basal granules together. 



Following treatment with osmic acid, the flagellum or cilium separates 

 into a bunch of unbranched fibrils. Recent studies with the electron 

 microscope reveal that the number of fibrils is remarkably constant, there 

 being nine strands spaced peripherally and two single strands lying centrally 

 in the cilium or flagellum (9 + 2 structure). The cilium is invested by a 

 membrane continuous basally with the cell membrane. The basic apparatus 

 common to all cilia and flagella comprises these eleven fibrillae plus their 

 basal granules (together with matrix and envelope), and it is this system 

 that is responsible for movement. One explanation for ciliary movement 

 postulates the following mechanism : the nine outer fibrils are contractile, 

 whereas the central pair is specialized for conduction. Rhythmical im- 

 pulses arising at one point in the basal body circulate around it, producing 

 propagated localized contractions in each of the peripheral fibrils (23, 41, 

 44, 129). 



Cilia are often associated together into larger vibratile units. When 

 conical in form these compound structures are known as cirri; when 

 organized as plate-like structures they are termed membranellae. The cirri 

 of Euplotes, for example, appear to consist of a series of cilia embedded in 

 a viscous matrix. The latero-frontal cilia on the gills of Mytilus consist of 



