State the reason for the following:
$(i)$ Nematodes have a pseudocoelom.
$(ii)$ Arthropods have an open circulatory system.
$(iii)$ Echinoderms are named as such.

(N/A) $(i)$ Nematodes are called pseudocoelomates because their body cavity is not fully lined by mesoderm; instead,the mesoderm is present as scattered pouches between the ectoderm and endoderm.
$(ii)$ Arthropods have an open circulatory system because their blood is pumped out of the heart into body cavities called sinuses or hemocoel,where it directly bathes the tissues and organs,rather than flowing through a closed network of vessels.
$(iii)$ Echinoderms are named so because they possess an endoskeleton of calcareous ossicles,which gives their body a spiny appearance (the term 'Echinodermata' is derived from Greek words meaning 'spiny skin').