Why are the eggs of most terrestrial animals surrounded by shells while those of aquatic species are not?

If you mean the eggs of animals like fish then fish do not need egg shells made from calcium because underwater there is no need for such a strong shell.

If you see frog eggs or fish eggs they will most likely be in a small water pouch.

Another thing reptiles such as crocs or turtles do not lay their eggs underwater because it would be difficult for their young to get out of it as the pressure of the water would complicate things. That is why you always see turtles being born on land.

Also animals like fish lack the reproductive organs needed to make such shells.

Then of course there are sharks and whales which give live birth to their young underwater. All mammals give live birth.

Although most sharks give live birth there are a few other sharks that do lay these types of marine eggs.



Here is a picture of a shark egg which is sometimes called a mermaid pouch;

http://images.search.yahoo.com/search/im...



Here is a normal fish egg;

http://images.search.yahoo.com/search/im...



and tadpole eggs;

http://images.search.yahoo.com/search/im...



hope this answered your question

Why are the eggs of most terrestrial animals surrounded by shells while those of aquatic species are not?
Things are land are cracked up while in the water they go with the flow
Reply:Egg shells are an evolutionary adaptation to the drier conditions on land. In an aquatic environment eggs are surrounded by water from which they obtain oxygen (which is dissolved in the water) and into which they pass waste products (such as carbon dioxide).



On land the developing embryo still needs water in which to develop and grow so a shell has been evolved. In reptiles (and the two living species of egg-laying mammals) this shell is leathery whilst birds have a more brittle shell.



Both act to contain a "miniature pond" inside the shell which acts the same way as the aquatic environment used to in underwater species. The shell is porous and allows oxygen and carbon dioxide to pass through but keeps the developing embryo protected in it's own personal aquatic environment.



In this way the shell performs several functions:-

+ contain the water the embryo needs in which to develop

+ keep the water from evaporating, thus preventing dehydration

+ protecting the embryo by providing a tough surrounding layer

+ permitting gaseous exchange
Reply:The shell serves to avoid the dehydration, problem that in water not is.

Ciao!
Reply:To prevent dessication and to protect from impact, predation, and infection.