I am not a biologist or really anyone with any authority on the matter. Just some guy who likes to read and think about all manner of subjects, so I cannot adequately explain anything here, but if you're interested in the why, it really boils down to the simplicity of morphological structures early in the development of life on earth, to more complex as evolution did its thing. That's not to say that evolution has a goal, just that added complexity often means greater advantages. Also, it isn't as though nothing similar to these creatures exist at all today. These basal forms were a prerequisite to the life we see in the oceans (and on land) today.

Definitely stay interested and read more about morphology and evolution in general! Fascinating stuff.