“When my husband died, because he was so famous and known for not being a believer, many people would come up to me – it still sometimes happens – and ask me if Carl changed at the end and converted to a belief in an afterlife. They also frequently ask me if I think I will see him again. Carl faced his death with unflagging courage and never sought refuge in illusions. The tragedy was that we knew we would never see each other again. I don’t ever expect to be reunited with Carl. But the great thing is that when we were together, for nearly twenty years, we lived with a vivid appreciation of how brief and precious life is. We never trivialized the meaning of death by pretending it was anything other than a final parting. Every single moment that we were alive and we were together was miraculous – not miraculous in the sense of inexplicable or supernatural. We knew we were beneficiaries of chance… That pure chance could be so generous and so kind… That we could find each other, as Carl wrote so beautifully in Cosmos, you know, in the vastness of space and the immensity of time… That we could be together for twenty years. That is something which sustains me and it’s much more meaningful… The way he treated me and the way I treated him, the way we took care of each other and our family, while he lived.

That is so much more important than the idea I will see him someday.

Above: Fastosarion brazieri, commonly known as the chameleon semi-slug, and an unidentified species of semi-slug from the genus Sheldonia

The term "semi-slug" is used to describe an intermediate stage of evolution as snails evolve into slugs. Nearly 1,000 different species of semi-slugs are known to exist, and these bizarre little creatures can be found on at least four continents.

Above: Fastosarion brazieri and Varadia amboliensis

Each species of semi-slug is technically still classified as a snail, but its shell is noticeably reduced, becoming more internalized as the species evolves. A semi-slug officially becomes a regular-slug once its shell is no longer visible at all.

As this article explains:

If life were simple, there would be snails and slugs. Snails carry their homes on their backs; slugs are naked and embarrassed. But life isn’t simple, so of course there’s secret option #3 – the semi-slug, a bizarre creature that sits exactly between the snail and the slug.

Above: genus Satiella and genus Euaustenia

This article also adds:

In contrast to snails that have an external shell large enough to accommodate the body, or slugs in which the shell is completely internal or absent, semi-slugs have an external shell, but the shell is too small to accommodate the animal’s entire body.

Above: Megaustenia siamensis

This process is known as limacization, and it's especially common in moist, low-calcium environments where a snail's shell may be more of a burden than a benefit:

Terrestrial slugs are not a monophyletic group, but a case of convergent evolution in which the slug form evolved from different lineages of land snails that gradually lost their shell through a process called limacization. Limacization resulted in adaptive radiation in land snail lineages, as slugs became adapted to diverse moist and protected spaces, such as crevices in rocks and wood debris. The loss of the shell also allowed for more movement and less calcium dependence, making slugs more successful as pests.

Above: Gaeotis nigrolineata, also known as the Puerto Rican semi-slug, has a neon green shell that is almost completely internalized, but the shell is clearly visible through the semi-slug's translucent body

Some semi-slugs have shells that are still opaque and largely visible, with the mantle (a patch of flesh) covering only the outer edges of the shell, while others have shells that are more significantly reduced, transparent, and/or concealed.

Above: Ibycus rachelae, commonly known as the green-shelled semi-slug, and a species of semi-slug from the genus Durgella

This topic was mentioned in my previous post about Ibycus rachelae, but I wanted to write a more detailed post about semi-slugs, because they're just so fascinating and weird.

Above: a black-and-white semi-slug from subfamily Sheldoniinae

Sources & More Info:

Australian Geographic: Meet the Semi-Slug, a Snail without a Home

Carnegie Museum of Natural History: What's So Good about Being a Slug?

Frontiers: Terrestrial Slugs in Neotropical Agroecosystems (PDF)

iNaturalist: Photos 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, & 13

The Shell-Makers (Introducing Molluscs): On Becoming Sluggish

Land Snails and Slugs of Sabah and Labuan, Malaysia: Semi-Slugs