12 



THE BRITISH NATURALIST. 



[January 



plasm of which these animals consist, and it seems probable that they 

 facilitate respiration. 



The absence of respiratory organs is noticeable in the division to which 

 the name zoophytes is given, and which includes the sponges and coral 

 animals or "insects " as they are sometimes erroneously called. 



In the next higher division, the worms, are included animals presenting 

 considerable variations of structure, and, as may be imagined, of respira- 

 tory apparatus. 



In many there is a water-vascular system which subserves respiration ; 

 this usually consists of two lateral trunks from which branches are given 

 off. In the simplest forms there is one cross-branch in each segment of the 

 body ; in other words there is one tube along each side of the body and a 

 connecting cross-tube for each segment of the body ; the water enters 

 this system of tubes by one or more external orifices ; the whole presents 

 a striking resemblance to the tracheal system of insects, which is really a 

 higher development adapted for aerial instead of aquatic respiration. 



In the earth worms there are neither gills nor water- vascular apparatus, 

 but the dense though thin investment of the body is perforated by minute 

 orifices through which the interchange of oxygen and carbonic acid takes 

 place. 



In many of the higher worms gills are developed which in some species 

 are disposed regularly in tufts along the sides of the body, as for instance 

 in the lug-worm (Arenicola piscatorium) ; in others, as Serpulae, which 

 secrete shelly coverings or tubes, the gill tufts are situated near the head, 

 and with it can be withdrawn into the tube when the animal is alarmed. 



The three next groups in the scale of animal life are molluscs, arthro- 

 pods, and echinoderms ; though, speaking broadly, all three are much 

 more highly specialized than the worms, yet in all there are plain evidences 

 of worm-like ancestry and relationships, either in the larval or adult 

 state of some of the members of each of the three widely separated groups. 



The way in which the transition from worm to mollusc was effected 

 is indicated by the Brachiopods, creatures which have risen above the 

 annelids and superficially resemble Lamellibranch molluscs, though they 

 are only distantly and collaterally related to them : they are marine 

 animals which have bivalve shells and a mantle similar to that possessed 

 by many molluscs. In this group i Brachiopoda) the mantle is a breath- 

 ing organ, though no true gills are developed. In true molluscs, distinct 

 respiratory organs may be absent, though usually either gills or a 

 pulmonary sac are developed ; the latter being present in the land 

 molluscs, of w T hich the common snail is a familiar example. The pul- 1 

 monary sac or lung is a development of the mantle. 



Under the name of Arthropods are grouped by far the greatest J 

 number of the species of animals which exist or have existed on the 

 earth. It is computed that four-fifths of the existing species of animals > 



