222 On the Structure and Functions of the Organs of Respiration. 



the whole of their internal canals, although we cannot detect the organisa- 

 tion by which this motion is iierformed. In their embryo state, however, 

 when they exist as genimules, we find them provided with cilia, in the same 

 manner as the infusorial animalcules. 



In passing upwards, through the classes of polypifera (true zoophytes) 

 and acaiepha (sea nettles), we find the function of respiration still per- 

 formed according to the same type, until we arrive at the echinodermata, 

 where we find tubes conveying water, ramifying throughout the interior of 

 the body' — the first appearance of an apparatus specially modified for the 

 aeration of the circulating fluid. 



In ascending through the series of articulated animals, from the simple 

 parasitic worms, to the liighly organised insects or Crustacea, we find the 

 respiratory apparatus assuming a more complicated form, and it is in this 

 series that we first meet with animals inhabiting the air. The extension 

 of the respiratory surface in air-breathing animals always takes place in- 

 ternally; though the converse, as we have just seen, does not always hold 

 good with regard to the inhabitants of the water. 



In the aquatic orders of the extensive class of annelides, we often find the 

 aerating organs in the form of delicate plumose tufts, beautifully radiating 

 from a centre, to form a fan-like expansion around the head, and frequently 

 displaying the most splendid variety of colours. There are few sights more 

 striking to the tropical naturalist, than the unexpected view of a bed of coral 

 in shallow water, having its surface scattered with the brilliant tufts of the 

 serpulae which have formed their habitations in it; the glowing and variegated 

 colours of which, when lighted up by the mid-day sun, and contrasted with 

 the sombre hues of the surrounding rocks, present an appearance compared 

 with which the most beautiful garden of carnations (which these animals 

 much resemble in form) sinks into insignificance. In another portion of 

 the class annelides, these ramified tufts are disposed at intervals along 

 the body of the animal, as in the common sand worm (arenicola piscatorumj 

 or in the sea mouse (halithcea aculeataj. A third form of the lespiratory 

 apparatus exists in those forms of the annelides adapted to live in air as 

 well as water, such as the leech or the earth worm ; here we lose the 

 external gills, and find in their place a series of small sacs, opening from 

 the sides by small orifices, termed stigmata, and extending into the interior 

 of the body ; presenting us with a simple but interesting condition of the 

 complex respiratory apparatus of insects. 



In the myriapodes, of which the centipede may be taken as an example, 

 the same structure exists, but in a less simple form, presenting us with the 

 intermediate stage between that of the air-breathing annelides, and of in- 

 sects. The respiratory apparatus of the latter class offers some beautiful 

 analogies with the corresponding system in plants. It consists of a vast 

 number of trachece, or air passages, which ramify in extremely minute 

 divisions throughout the smallest parts, and which open by stigmata, or 



