SEGMENTATION I93 



glands, gonads, coelomoducts, nephridia, ganglia, and nerves may 

 be repeated in almost indistinguishable units down the body. 

 Such a state is found in many polychaetes. The earthworm has 

 fairly complete segmentation, but the gonads are confined to one 

 or two segments and the distribution of the accessory sexual 

 organs such as seminal vesicles makes many of the anterior seg- 

 ments distinct. The segmentation of the crayfish and of insects is 

 more superficial, and there is little visible internally, but the 

 continuous exoskeleton makes external jointing functionally 

 important. The embryos of vertebrates have a conspicuous and 

 fundamental segmentation ; in the fishes much of this persists in 

 the adult, but in the other classes practically no traces of it are 

 left. Even the vertebrae, which appear to be obviously segmental, 

 do not correspond to the segments of the embryo. 



Many segmented animals have the first few segments much 

 modified to form a head. This process is known as cephalisation ; 

 it occurs to some extent in Nereis, and is conspicuous in the 

 insects and vertebrates. The changes generally include the 

 development of special limbs or other structures around or in the 

 mouth ; the specialisation of sense cells and their association with 

 other types of cell to form conspicuous organs such as eyes ; and 

 internally the concentration of nervous tissue into a brain. 

 Cephalisation is generally regarded as an inevitable result of 

 always moving forwards, and it certainly seems reasonable that 

 that part of the body which first meets food and first learns about 

 the outside world should be specialised to eat and see. It is only, 

 however, in segmented animals that cephalisation is highly 

 developed ; the most highly organised unsegmented animals, the 

 molluscs, do indeed have something of a head, but it is remarkable 

 that in many cephalopods, where it is best developed, it is pretty 

 well in the middle of the body, and that these creatures move 

 backwards much more rapidly than forwards. The octopus, alone 

 amongst animals, bears any resemblance to Shakespeare's ' Men 

 whose heads, Do grow beneath their shoulders '. 



OSMOTIC REGULATION 



It is a general property of living membranes that the}' are 

 semipermeable, that is, allow some substances, especially water, to 

 pass through them much more rapidly than do others. A result of 

 this property is that when they separate two aqueous solutions 



