Regarding the interrelationships of the various types that we have described, and their respective importance with reference to the descent of man, opinions are somewhat divided. Some believe the Ctenophora to have been derived from the Medusa form, but the more probable view seems to be that they have evolved separately from some earlier and more primitive type than any existing Cœlenterate, and that their ancestors have all been free-swimming and ciliated. Now the Ctenophora are considered, on good grounds, to be somewhat nearly akin to the lowest worms, and thus to stand fairly close to the main line of Evolution. If this view be correct, the whole group of existing Cœlenterates forms a side branch of the Evolution tree. This fact, however, does not take away the importance of the group in relation to the theory of the descent of the higher animals, for the Cœlenterates have certainly retained many of the characters which were possessed by the direct ancestors of man, such, for instance, as the simple digestive cavity, the primitive type of body, consisting of two cell layers, the diffuse and elementary nervous system, and the radial arrangement of parts. Moreover, the course of Evolution in the group, leading from the Hydra to the sea-anemone and the Medusa, has probably been in many respects parallel to that which started from some primitive extinct form, and led up to the Ctenophora. Therefore the study of the group has thrown much light on the earlier history of the animal world. Regarding the age of the group, it may be mentioned that fossil corals, etc., are found, along with Crustaceans and Molluscs, in the earliest known fossil-bearing beds, belonging to the Cambrian age. CHAPTER III THE WORMS AND SOME OF THEIR POSTERITY The somewhat miscellaneous collection of animals that have been thrown together and termed _worms_ is of the greatest importance for our theory of descent. Indeed, it seems probable that all of the four great groups which we have yet to mention have descended directly from worm ancestors. This, at all events, is the view of Haeckel, although it must be admitted that many other theories have been proposed. Nor can it be taken as a matter for surprise that agreement concerning this part of our history should be hard to reach, for the difficulties which are met in it are many and perplexing.