Cnidoblasts are special structures containing stinging capsules. The poisonous substance from the stinging capsule is used to kill the prey.

Osculum; the water enters through ostia and moves out through osculum.

The notochord is mesodermally derived structure formed on the dorsal side during embryonic development in some animals.

In coelomates the mesoderm is lining the body cavity, while in acoelomates the mesoderm is present in small pouches.

Reptilia are crawling animals and their skin is hard and dry to withstand extreme temperatures.

Blood-filled cavity in arthropods is haemocoel and lateral appendages in aquatic annelids are parapodia.

(i) Two - chambered heart - Pisces
(ii) Three - chambered heart - Amphibians and reptiles except crocodiles
(iii) Four-chambered heart - Crocodiles, aves and Mammalia 

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.

Differences between intracellular and extracellular digestion are
(i) Intracellular digestion occurs inside the living cells with the help of lysosmal enzymes. Food particle taken in through endocyotosis It forms a phagosome, which fuses with a lysosome. The digested material passes into the cytoplasm. The undigested matter is thrown out by exocytosis. It occurs in Amoeba, Paramecium, etc.
(ii) Extracellur digestion In case of coelenterates, digestion occurs in gastrovascular cavity. This cavity, has gland cells which secrete digestive enzymes ove the food. The partilly digested framented food particles are ingested by nutritive cells. It occurs in Hydra, Aurelia and higher animals, etc.

Digestive system having only a single opening to the outside of the body that serves as both mouth and anus, called incomplete digestive system while a complete digestive system has two separate openings, i.e mouth and anus.

In Platyhelminthes, true segmentation is absent but pseudosegmentation is seen,which is reduced by repeated budding or strobilation.

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.

Gas bladder or air bladder is a gas-filled sac present in fishes. It helps in maintaining buoyancy. Thus, it helps fishes to ascend or descend and stay in the water current.

Sycon (sponge) possesses canal system and bear spicules.

tapeworms

Annelida is the first phylum showing a closed circulatory system. In open circulatory system blood is directly supplied to tissues. In closed circulatory system special tubes are in place to circulate blood. This is more efficient in circulating blood in larger and complex organisms. So all higher organisms have closed circulatory system.

Features of cartilaginous fishes
(i) Cartilaginous endoskeleton is present
(ii) These are mostly marine, poikilothermic animals
(iii) Notochord is persistent throughout the life
(iv) Placoid scales are present
(v) Heart is two-chambered with one auricle and ventrical
(vi) Tail fin is always heterocercal

Annelids are considered evolutionary precursors of molluscs as they do not have metameric segmentation.However, Neopilina a living fossil shows metameric segmentation.Besides,its trochopore larva, it has structural similarities to annelid trochopore larva.These two evidences suggest that molluscs have descended from annelids. 

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)

Molluscs have diverse habitats
(i) Marine Chiton,Sepia,Octopus
(ii)Fresh water unio and Pila
(iii)Terrestrial Land snails
(iv) Parasites Glochidium larva

Phylum - Coelenterata (Cnidaria)

Habit and Habitat. They are aquatic, mostly marine, sessile or free-swimming, radially symmetrical animals. The name cnidaria is derived from the cnidoblasts or cnidocytes (which contain the stinging capsules or nematocytes) present on the tentacles and the body. Cnidoblasts are used for anchorage, defense and for the capture of prey.
Key Structural Features. Cnidarians exhibit tissue level of organisation and are diploblastic. They have a central gastro-vascular cavity with a single opening, hypostome. Digestion is extracellular and intracellular. Some of the cnidarians, e.g., corals have a skeleton composed of calcium carbonate.
Basic Body Forms. Cnidarians exhibit two basic body forms called polyp and medusa. Polyp is a sessile and cylindrical form like Hydra, Adamsia, etc. whereas,
medussa is umbrella-shaped and free-swimming like Aurelia or jelly fish. Those cnidarians which exist in both forms exhibit alternation of generation
(Metagenesis), i.e., polyps produce medusae asexually and medusae form the polyps sexually (e.g., Obelia).
Examples: Physalia (Portuguese man-of-war), Adamsia (Sea anemone), Pennatula (Sea-pen), Gorgonia (Sea-fan) and Meandrina (Brain coral).

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.