Means of infection . Use the chart that you started in the chapter on phylum Porifera to add perspective to vour study of the acoelomate animals. To increase your awareness of the overall pattern of relationships, fill in the spaces for the new traits added in this chapter for the phyla from previous chapters, for example the radiate animals and the sponges. If you continue to develop this large chart, it will be very useful in preparing for the final exam. . Nemertea Gnathostomulida .
Larval forms are of importance because they often indicate degrees of relationships between phyla.
In later phyla, there will considerable discussion about the relationships within a phylum and between a variety of phyla based on the similarities of larval forms. This is a good place to begin to understand how this idea works. Up to this point the majority of the animals have had radial symmetry if any symmetry was present at all. These radially symmetrical animals are thought to have evolved from a planuloid larval form that was bilaterally symmetrical. When animals such as the Cnidaria settled to a sessile life style or became free floating, radially arranged sense organs were more advantageous than a head.