Arachnids 9th Rank

These are animals:

• breathing by narrow trachea
• not undergoing any transformation
• with articulated limbs
• with eyes in their heads

The ninth rank belongs to the arachnids.

They have so many affinities with the crustaceans, that we are always compelled to bring them close together.

Nevertheless, they are clearly different from the crustaceans.

They display the first example of a respiratory organ inferior to gills, because we do not ever come across it in the animals which have a heart, arteries, and veins.

In fact, the arachnids breathe only by stigmata and air trachea which are respiratory organs analogous to those of insects.

But these trachea, instead of extending themselves throughout the entire body, like those in insects, are circumscribed in a small number of vesicles.

This fact shows that nature concludes, in the arachnids, the method of respiration which she had to use before establishing gills, just as she concluded, in the fish or in the last reptiles, what she had had to make sure of before she could form a true lung.

If the arachnids are clearly distinguished from the crustaceans, because they do not breathe at all by gills but by very narrow air trachea, they are also clearly different from the insects.

It would be just as inconvenient to group them with the insects, whose classic characteristic they do not have, and from whom they differ especially in their interior organic structure as to mix up the crustaceans with the insects.

The arachnids have some important similarities to insects.

But they are essentially distinct because:

1. They never undergo a metamorphosis.

They are born with the shape and all the parts which they must always retain and, consequently, they always have eyes in the head, articulated limbs, things arising out of the nature of their interior organic structure.

In this they are very different from the organic structure of insects.

2. In the arachnids of the first order (pedipalp-arachnids) we begin to notice the traces of a system of circulation (3).

3. Their respiratory system is very different from those of insects.

Their trachea is limited to a few vesicles. They are not made up of very numerous canals to the air extending throughout the animals’ bodies, as we see with the trachea of insects.

4. The arachnids reproduce several times in the course of their lives, an ability which the insects lack.

This is why it is wrong to combine arachnids and insects in the same class.

Their authors only focused on:

• the limbs
• the crustaceous skin covering them

This would happen if we took the scaly teguments of the reptiles and fish and thus combined them in the same class.

As for the general degradation in organic structure which we are looking into as we move through the entire animal scale, that is very clearly evident in the arachnids.

Arachnids breathe through an organ less well developed with respect to structural improvements than the lung and even gills and have only the preliminary traces of a circulation which does not yet appear complete, they confirm, on their part, the continuing degradation in question.

This degradation is noticed even in the series of species brought together in this class. For the arachnids with antennae or of the second order are very different from others, are very inferior to them in the development of their organic structure and are very close to insects.

Nonetheless, they differ from insects in that they do not undergo any transformation.

Since they never fly up into the air, it is very probable that their trachea do not generally extend to all parts of their bodies.