001 - title image

The development of Physella,
a Freshwater Snail
2nd part


Most of the pictures were taken through a 10x objective and a 10x ocular , with a resolution of 640 x 480 px. (if there is no contrary indication in the picture caption). The 480 px side of the pictures measure exactly 1000 microns. Darkfield and various Rheimberg filters were used to give more contrast and definition to the pictures.

Part 1 of series.


Almost all the freshwater gastropods put their eggs in jelly-like masses adhered to different surfaces (stones, leaves and stems of submerged plants). At the aquarium they choose generally the glass walls where they are easily visible like little discs, in some species, or with a horseshoe shape like in the one shown below.

010 - ootheca

This picture was taken using the trick that I name " The Köhler microscope" using the 10x objective. The longer side of the picture is more or less 1 cm long.

The picture shows a gelatinous ootheca that contains 90 transparent eggs, which allows us to easily follow the embryo development. The eggs recently laid have a cytoplasm full of yolk and with a big nucleus. In just a short time the egg  divides into two cells, soon into four, 8, 16 cells and form at the end a morula, that is to say, a spherical and compact mass of more or less spherical cells (of course the name comes from the similarity of this embryo with the fruit of the Morus sps. trees, the mulberries). From now on, it is more difficult to follow the detailed development, because the most important changes happen in the interior of the embryo, and it would need the aid of a histologist to identify them.

011 - young embryo
A very young embryo.

 We know that the morula originates as a hollow, more or less spherical blastula, the wall of which is turned inward to form a two layered gastrula. Posterior development produces a new cell layer between the first two and the embryo quickly generates a nervous cord and begins to differentiate the organ systems characteristic of the adult.

012a . symmetrical embrio-1
012b - symetrical embrio.2
An embryo prior to the start of the "torsion" (see text).

 An interesting detail of the development of the mollusks is that they start as “right” and symmetrical, but soon begins a process of asymmetrization and torsion of the visceral mass, at the same time that this is being covered by the segregated calcareous shell.

013-torsion, 1
014 - torsion, 2
 Two embryos that have started the torsion of the visceral mass hump over the flat foot.

The torsion causes the atrophy of the organs in the left side of the body, if torsion is to the right, or on the right side if it is to the left.


015 - mobile embryos
Embryos start to move inside the shell.

When growing, the shell follows the helical torsion process generating the gastropods characteristic spiral. As is logical, with the growth of each successive turn the shell constructs a center axis the “columella”. A beautiful picture of this structure is seen in fig. 33.

016 - stretching embryo

The embryo is starting to secrete its shell over the visceral hump.The head is in frontal view, with mouth in the center, and the primordium of the left tentacle on the uper side.You can see the thickening at the extreme right of the embryo.

These are two pictures of other more developed embryos.

017 - radula embryo
018 - camara embryo
The digestive system is well developed in this snail embryo.
One can detect the developing radula as a clear band starting at the mouth.
The dark spot in the visceral mass is the developing lung.
You can see also the primordium of one eye.

In just a short time the little snails begin to move within the egg, as the shown in the following animations.

019 - active embryo 1
020 - active embryo 2

If you look closely at some of the more sluggish embryos, you can see the beat of the heart of the growing embryo.

021 - hearth beating

Soon they break through the egg’s shell and leave it. The youthful individuals are beautiful photogenic subjects.

023 - hatching 1
hatching 2

 Two pictures of the embryo breaking through the egg shell.

025 - empty shell
 One empty shell.

026 - newborn 1
027 - newborn 2
Exploring the world.

028 - dorsal view
029 - -dorsal, labels

030 labeled shells
031 - empty shells
Morphology of the conch. The shells are 1 cm high.

032- columella
This one picture was a cut-out of a stereoscopic pair. It depicts the beauty of a radioscopy of a busycon shell. Of course I don't know anything that can show mor eclearly the genesis of the columnella. Those that want to see the original must visit this site:

       Peter Abraham .- Stereo X-rays of sea shells

Comments to the author, Walter Dioni , are welcomed.


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