Crustaceans

Animals having a body and articulated limbs, a crustaceous skin, a circulation system, and breathing by gills.

At this point we enter into the numerous series of animals in which the body and especially the limbs are articulated and the integuments are crustaceous, horny, or coriaceous.

The solid or strong parts of these animals are all on the outside.

Nature created the muscular system very shortly before the first animals of this series and required points of attachment in solid parts to give the system energy, she was obliged to establish the method of articulations to make movement possible.

All the animals united by a similar method of articulation have been considered by Linnaeus and, following him, as forming only one single class, to which was given the name insects.

But we finally recognized that this large series of animals displays several important divisions which it is essential to distinguish.

The class of crustaceans, which has been confused with the class of insects (although all the ancient naturalists) had always separated it, is a division indicated by nature and essential to preserve.

It must follow immediately after the annelids and occupy the eighth rank in the general series of animals. The analysis of its organic structure requires that. There is nothing at all arbitrary in this matter.

The crustaceans have a heart, arteries and veins, a transparent, almost colourless circulating fluid, and all breathe by true gills. That is incontestable and will always embarrass those who continue to rank them among the insects because they have articulated limbs.

If, because of their circulation and the respiratory organs, the crustaceans are clearly distinguished from the arachnids and the insects and if, consequently, their rank is clearly superior, nevertheless they share with the arachnids and insects this lower feature of organic structure, with respect to the annelids, that is, they are part of the series of animals with articulated limbs, a series in which we see the circulation system going away and disappearing.

Consequently, the heart, the arteries and veins, and even the respiration by the system of gills similarly is lost.

Thus, the crustaceans confirm, in their turn, the degradation maintained in the organic structure, in the direction we are moving through the animal ladder.

The fluid which circulates in their vessels is transparent and almost without substance, like that in the insects, and demonstrates once more this degradation with respect to them.

As to their nervous system, it consists of a very small brain and a longitudinal marrow with ganglia, a characteristic impoverishment of this system, which we see in the animals of the two preceding classes and the two which follow, for the animals of these classes are the last ones in which the nervous system is still present.

It is in the crustaceans that the last traces of the gill organ has been perceived. After them, it does not reappear in any animal.

Observations

At this point the existence of a true system of circulation comes to an end, that is, a system of arteries and veins which makes up part of the organic structure of the most improved animals and which all the structures of all the preceding classes of animals possess.

The organic structure of the animals which we are going to discuss is thus more imperfect still that that of the crustaceans, who are the last in which the circulation is well manifested.

Thus, the degradation in the organic structure continues in a clear manner, because to the further one moves ahead in the series of animals, all the features of resemblance between the organic structures of the animals we are considering and that of the most improved animals is successively lost.

Whatever the nature of the movement of fluids in the animals of the classes which we are going to go through, this movement works by less active means and always in a slower way.