Differences between the direct and indirect development are as follows.
Direct development : In case direct development the life cycle of an adult individual lacks any larval stage in its life cycle. The adult give rise to young ones which directly develop into the adult.e.g, mammals
Indirect development : It is present mostly in case of lower animals, the adult individual gives rise to eggs mostly which develop into the adult after being passing through serval larval stages, e.g, echinoderms Ascaris, cockroach have prolonged larval stages in their life cycle.

Amphids are pits commonly having olfactory chemoreceptors.
Phasmids are glandulo-secretory porous sensory structures.

Different exoskeletal structures present in different animals are 

All animals that possess a notochord are called chordates. Vertebrata is a sub-phylum under Phylum Chordata and are hence, "vertebrate" is not synonymous to "chordate". Vertebrates possess notochord during the embryonic period. The notochord is replaced by a cartilaginous or bony vertebral column in the adult.

Animals possessing coelom are called coelomates, e.g, annnelids, molluscs, arthropods, etc.

Most bony fishes have cycloid or ctenoid scales. Both cycloid and ctenoid scales consist of an outer layer of calcium and an inner layer of connective tissue.

In the animal kingdom, there are two broad phyla that exhibit radial symmetry: One of these is cnidaria, which include jellyfish, anemones, and corals. Jellyfish exhibit radial symmetry in four points around its center. The other phylum is echinodermata, such as sea stars, urchins, and sea cucumbers.

It is the segmentation that simultaneously divides body both externally and internally and is also called metamerism.

General features of roundworms are
(i) The body is cylindrical and tapering at both ends.
(ii) No segmentation.
(iii) Bilateral symmetry
(iv) They have organ system level of organisation
(v) The anterior end does not form a distinct head
(vi) There are no locomotory appendages in roundworms
(vii) The body wall consists of firm, non-living, resistant cuticle, epidermis and muscle layer.
(viii) These are pseudocoelomates.
(ix) Hydroskeleton is present.
(x) Digestive tract is complete.Respiration occurs by body surface.
(xi) Circulatory system is undeveloped.Excretion occurs through gland cells.
(xii) Sexes are separate.Sexual dimorphism is present.  Fertilisation is internal.There is no asexual reproduction.
(xiii)Development is direct.

Various excretory organs found in different animals are 

The molluscs have advancement over annelids, as they have distinct head, better sense organs and gills or lungs for respiration.

The body of sponges contains a supporting exoskeleton. It is made up of either spicules or spongin fibres or both. Spongin fibres are protein fibres. They are either highly branched or show anastomosis. Spicules are calcareous or siliceous with hard spine-like structures.
Depending on the branching the spicules are
(i) monoaxons (single axis).
(ii) triaxons (three axis forming six rays of hexactinal spicules).
(iii) tetraxons (four rays or tetra actinal).
(iv) polyaxons (several rays)

Different modes of respiration in animals are given in the table below.