Of these, four were for the first time defined, and the others restricted. It will be noticed that he separates the Radiata (Radiaires) from the Polypes. His “Radiaires” included the Echinoderms (the Vers echinoderms of Bruguière) and the Medusæ (his Radiaires molasses), the latter forming the Discophora and Siphonophora of present zoölogists. This is an anticipation of the division by Leuckart in 1839 of the Radiata of Cuvier into Cœlenterata and Echinodermata.
The “Polypes” of Lamarck included not only the forms now known as such, but also the Rotifera and Protozoa, though, as we shall see, he afterwards in his course of 1807 eliminated from this heterogeneous assemblage the Infusoria.
Comparing this classification with that of Cuvier[121] published in 1798, we find that in the most important respects, i.e., the foundation of the classes of Crustacea, Arachnida, and Radiata, there is a great advance over Cuvier’s system. In Cuvier’s work the molluscs are separated from the worms, and they are divided into three groups, Cephalopodes, Gasteropodes, and Acephales—an arrangement which still holds, that of Lamarck into Mollusques céphalés and Mollusques acéphalés being much less natural. With the elimination of the Mollusca, Cuvier allowed the Vers or Vermes of Linné to remain undisturbed, except that the Zoöphytes, the equivalent of Lamarck’s Polypes, are separately treated.
He agrees with Cuvier in placing the molluscs at the head of the invertebrates, a course still pursued by some zoölogists at the present day. He states in the Philosophie Zoologique[122] that in his course of lectures of the year 1799 he established the class of Crustacea, and adds that “although this class is essentially distinct, it was not until six or seven years after that some naturalists consented to adopt it.” The year following, or in his course of 1800, he separated from the insects the class of Arachnida, as “easy and necessary to be distinguished.” But in 1809 he says that this class “is not yet admitted into any other work than my own.”[123] As to the class of Annelides, he remarks: “Cuvier having discovered the existence of arterial and venous vessels in different animals which have been confounded under the name of worms (Vers) with other animals very differently organized, I immediately employed the consideration of this new fact in rendering my classification more perfect, and in my course of the year 10 (1802) I established the class of Annelides, a class which I have placed after the molluscs and before the crustaceans, as their known organization requires.” He first established this class in his Recherches sur les corps vivans (1802), but it was several years before it was adopted by naturalists.
The next work in which Lamarck deals with the classification of the invertebrates is his Discours d’ouverture du Cours des Animaux sans Vertèbres, published in 1806.
On page 70 he speaks of the animal chain or series, from the monad to man, ascending from the most simple to the most complex. The monad is one of his Polypes amorphs, and he says that it is the most simple animal form, the most like the original germ (ébauche) from which living bodies have descended. From the monad nature passes to the Volvox, Proteus (Amœba), and Vibrio. From them are derived the Polypes rotifères and other “Radiaires,” and then the Vers, Arachnides, and Crustacea. On page 77 a tabular view is presented, as follows:
- Les Mollusques.
- Les Cirrhipèdes.
- Les Annelides.
- Les Crustacés.
- Les Arachnides.
- Les Insectes.
- Les Vers.
- Les Radiaires.
- Les Polypes.
It will be seen that at this date two additional classes are proposed and defined—i.e., the Annelides and the Cirrhipedes, though the class of Annelida was first privately characterized in his lectures for 1802.
The elimination of the barnacles or Cirrhipedes from the molluscs was a decided step in advance, and was a proof of the acute observation and sound judgment of Lamarck. He says that this class is still very imperfectly known and its position doubtful, and adds: “The Cirrhipedes have up to the present time been placed among the molluscs, but although certain of them closely approach them in some respects, they have a special character which compels us to separate them. In short, in the genera best known the feet of these animals are distinctly articulated and even crustaceous (crustacés).” He does not refer to the nervous system, but this is done in his next work. It will be remembered that Cuvier overlooked this feature of the jointed limbs, and also the crustaceous-like nervous system of the barnacles, and allowed them to remain among the molluscs, notwithstanding the decisive step taken by Lamarck. It was not until many years after (1830) that Thompson proved by their life-history that barnacles are true crustacea.
In the Philosophie zoologique the ten classes of the invertebrates are arranged in the following order: