Difference between metamerism and paedo metamerism
Answers
Metamerism is segmentation of body into somites or metameres. Pseudometamerism occurs in cestodes in which every segment is independent of the other and contains complete set of organs that have no connection with organs in other segments. During growth new segments are added in front, in the neck region and hence the posterior-most body segment is the oldest one and the anterior segments are younger.
In true metamerism, there is a serial repetition of homologous organs, like nephridia, nerves, muscles, reproductive organs, appendages etc. in each segment but these organs function in coordination with the others. All segments are integrated into a single functional unit. In true metamerism new segments are added in front of the last segment called pygidium. Hence posterior segments are younger as compared to the anterior ones. Coelom is divided by intersegmental septa into compartments, each of which can be regulated independently of the others.
Truly segmented animals typically have an anterior acron and posterior pygidium and various intermediate segments called metameres or somites. In higher invertebrates, such as arthropods, metamerism provided an opportunity for specialization of segments into head, thorax and abdomen and serially repeated organs could be specialised resulting in rapid evolution.
EVOLUTION OF METAMERISM
It is believed that during the course of evolution metamerism evolved three times independently for different purposes as mentioned below.
EVOLUTION OF PSEUDOMETAMERISM
Pseudometamerism occurs in cestodes such as tapeworms. There are two theories to explain its origin.
FISSION THEORY