Evolution 



cal nature of the water. The Nemertines possess a peculiar 

 organ in a snout or proboscis, which they can protrude or withdraw 

 into a special sac. The snout is armed with a sharp sting, and 

 forms an effective weapon whether against the creature's enemies 

 or its prey. 



About this stage of Evolution, the exact point being some- 

 what difficult to fix, there appears the body cavity. This, which 

 is altogether distinct from the digestive cavity, is a familiar 

 feature of the anatomy of the higher animals. In it are suspended 

 the heart and lungs and the whole of the digestive organs and 

 glands. The question of the origin of the body cavity and the 

 blood system is a very difficult one, and a thorough theoretical 

 discussion would take us too far. 



Before proceeding to the question of the origin of the verte- 

 brates, we may pause briefly to consider the other groups to 

 which the worms appear to have given rise. First of these we 

 may take the Echinoderms, which include the well-known star- 

 fishes and sea-urchins, and the very beautiful feather stars. As 

 already indicated, it is believed that the radial symmetry, which 

 is so characteristic of this group, is not a primitive feature, but 

 that, in fact, the Echinoderms are descended from bilaterally 

 symmetrical ancestors. One reason for this view is that the 

 larval or immature form is always markedly bilaterally sym- 

 metrical. In an ordinary star-fish, which we may take as typical 

 of the group, the mouth is in the middle of the lower aspect, and 

 the excretory opening of the digestive cavity in the upper side 

 just opposite. There is no blood system, or excretory organs, 

 and no concentration of nerve cells into any form of brain. Eyes, 

 however, are present, and sensitiveness to light may be easily 

 demonstrated. The most remarkable feature of the group is the 

 water- vascular system, consisting of a series of radial canals, 

 one in each ray, which join a circular one situated in the central 

 portion of the body. The system of canals communicates with 

 the exterior by means of a sieve-like plate on the upper surface, 

 and it is kept full of water by the continual pumping action of 

 cilia on the walls of the tube which leads down from the sieve plate. 



64 



