Dear Valerie,
I can give you a small answer to a very big question. Firstly I can't really see the difference in the two alternatives you ask about - to diverge they would  need first to be together.

The important first thing to realise is that a 'slug' is a body shape or 'grade' of organisation not an evolutionary group. For example sharks, bony fish, dolphins are all 'fish' in the sense that they all have a fish grade of organisation, but they are not closely related to each other in evolutionary terms. The slug shape has independently evolved many times in the gastropod molluscs from shelled snails. Have a look at the 'What is a slug ?' Fact Sheet. We still have much to learn about the evolution of gastropods, but the general consensus at present is that the Opisthobranchs and Pulmonates - that is the animals that make up the sea slugs and the land snails (& slugs) - have a common ancestry, being descended from a special group of marine snails with peculiarities in their nervous system, larval shell, radula and mantle cavity, which set them apart from all other marine snails.

So what we can say is that sea slugs and land slugs have a common ancestor, but the  huge diversity we see today is the result of many evolutionary events which include the independent development of the slug form many times.

Best wishes,
Bill Rudman

Dear Steev,
I have been meaning to read that book but have yet to find time so I'll make a guess at what he was talking about. I suspect he was referring to Slime Moulds [Slime Molds] which are a group of primitive organisms that used to be considered Fungi, but are now placed in the Kingdom Protista. One group of these, the cellular slime moulds, spend much of their life as single cells, but they have this amazing ability to aggregate together - when they get the urge - to form a 'slug' which can crawl along. The slug usually forms into a reproductive body, often raising itself from the substrate to form a little fruiting body.

Here are a couple of websites with information. The second one has some fascinating short movies.
http://www.hiddenforest.co.nz/slime/index.htm
http://cosmos.bot.kyoto-u.ac.jp/csm/movies.html

Are they 'slugs'? The term slug usually refers to the relatives of snails, both on land and in the sea, which have lost their shells during their evolution. However, as is the way with most common terms, 'slug' is widely used to mean any slug-like organism.

Best wishes,
Bill Rudman

Dear Amanda,
Have a look at the page on how snails became slugs. It will give you links to other relevant pages. The basic point is that 'slugs' have evolved many times from snails through the gradual downgrading in importance of the external shell, until finally the shell is completely lost in the adult, although often still present in the larvae. Of all the sea slug groups, it is only in the nudibranchs that the shell is absent in all adults. In other groups, such as the cephalaspidean 'Bubble-shells', such as Pupa coccinata, an external shell is often present, while in the sacoglossans, we have a continuum from animals with an external shell, such as Volvatella ayakii to 'nudibranch'-like species, such as Stiliger smaragdinus, in which there is no trace of a shell.
Best wishes,
Bill Rudman

Dear Stephaine,
Sea Slugs are specialised snails, and many of them, such as the nudibranchs, have 'lost' their shells. If you go to the What is a Slug? page you will find some information and links to other pages which discuss this topic.

Snails are molluscs, a Phylum of animals that also includes squid, octopus, and clams. Fish on the other hand are chordates (including vertebrates), a Phylum which includes all animals with a backbone - fish, frogs, birds, mammals etc.
Best wishes,
Bill Rudman

Dear LV,
I really can't deal with snails and slugs worldwide but from your description I suspect you may have Limax maximus of which there is a photo on the "what isn't a sea slug" page.

Often the back half of the body has stripes rather than spots. We call it the Leopard Slug in Australia. It is a European species which has been introduced to many parts of the world. It is more of a carnivore than a herbivore, preferring cat and dog food, to more natural foods.

If your slug doesn't look like the photo then I suggest you try your local museum. As much as I admire your attitude, I really haven't the time, or the knowledge, to take on all the world's molluscs.
Best wishes,
Bill Rudman.

Dear Jaykada,
I'm afraid there is not a scientific name for a slug. Have a look at the pictures and my comments at the top of this page.

Essentially a "slug" is a snail that has lost its shell, not through carelessness, but in an evolutionary process that has occurred many times amongst the gastropod molluscs. Both in the opisthobranch sea slugs and the pulmonate land snails there are many quite unrelated families and superfamilies which have independently begun the process of "losing the shell". Gradually the shell becomes smaller, and less heavily calcified until its reaches the stage where it no longer can protect the animal from attack or from environmental problems such as drying out.

At this stage the animal has to evolve other means of defence. Land slugs produce heavy mucus from their skins to prevent desiccation (drying out) and they are often well camouflaged, perhaps as a protection from attack. Most also become nocturnal, as they are less likely to be seen by predators or dry out at night. The many different ways opisthobranchs have evolved to protect themselves can be found in the Sea Slug Forum. Have a look at Opisthobranch Defence Mechanisms.
Bill Rudman.