The History of Creation, Vol. 2 (of 2) / Or the Development of the Earth and its Inhabitants by the Action of Natural Causes - Ernst Haeckel

The result of these structural dispositions for the history of the pedigree of Molluscs, which is confirmed by palæontology, is that Lamp-shells stand much nearer to the primæval root of the whole tribe of Molluscs than do the Otocardia. Probably Mussels and Snails developed as two diverging branches out of Molluscs, which were nearly akin to the Lamp-shells.

Mussels, or Plate-gills (Lamellibranchia), possess a bivalved shell like the Lamp-shells. In the latter, one of the two valves covers the back, the other the belly of the animal; whereas in Mussels the two valves lie symmetrically on the right and left side of the body. Most Mussels live in the sea, only a few in fresh water. The class is divided into two sub-classes, Asiphonia and Siphonida, of which the latter were developed at a later period out of the former. Among the Asiphonia are Oysters, mother-of-pearl Shells, and fresh water Mussels; among the Siphonida, which are characterized by a respiratory tube, are the Venus-shells, Razor-shells, and Burrowing Clams. The higher Molluscs seem to have developed at a later period out of those without head and teeth; they are distinguished from the latter by the distinct formation of the head, and more especially by a peculiar kind of tooth apparatus. Their tongue presents a curious plate, armed with a great number of teeth. In our common Vineyard Snail (Helix pomatia) the number of teeth amount to 21,000, and in the large Garden Slug (Limax maximus) to 26,800.

SYSTEMATIC SURVEY

Of the 4 Classes, 8 Sub-classes, and 21 Orders of Molluscs.

Classes of Molluscs.	Sub-classes of Molluscs.	Orders of Molluscs.		Systematic Name of the Orders.
I. Molluscs without head or teeth: Acephala or Anodontoda.

I. Lamp-shells Spirobranchia or Brachiopoda	I. Ecardines Hinge-less	1.	Stalked	1.	Lingulida
	I. Ecardines Hinge-less	2.	Flattened	2.	Craniada

	II. Testicardines Hinge-less	3.	Fleshy armed	3.	Sarcobrachia
	II. Testicardines Hinge-less	4.	Calcareous-armed	4.	Sclerobrachia

II. Mussels or Plate-gills Lamellibranchia or Phyllobranchia	III. Asiphonia Mussels without respiratory tubes	5.	One-muscled	5.	Monomya
		6.	Uneven-muscled	6.	Heteromya
		7.	Even-muscled	7.	Isomya

	IV. Siphonida Mussels with respiratory tubes	8.	Round-mantled	8.	Integripallia
		9.	Ray-mantled	9.	Sinupalliata
		10.	Tube-mussels	10.	Inclusa

II. Molluscs with head and teeth: Cephalophora or Odontophora.

III. Snails Cochlides or Gasteropoda	V. Stump-headed Perocephala	11.	Tube-snails	11.	Scaphopoda
	V. Stump-headed Perocephala	12.	Butterfly-snails	12.	Pteropoda

	VI. Large-headed Delocephala	13.	With hind gills	13.	With hind gills
		14.	With fore gills	14.	Prosobranchia
		15.	Swimming-snails	15.	Heteropoda
		16.	Beetle-snails	16.	Chitonoida
		17.	Snails with lungs	17.	Pulmonata

IV. Cuttles or Poulps Cephalopoda	VII. Chamber-Poulps with four gills Tetraranchia	18.	Pearl boats	18.	Nautilida
	VII. Chamber-Poulps with four gills Tetraranchia	19.	Ammon's horns	19.	Ammonitida

	VIII. Ink-Poulps with two gills Dibranchia	20.	Ten-armed	20.	Decabrachiones
	VIII. Ink-Poulps with two gills Dibranchia	21.	Eight-armed	21.	Octobrachiones

PEDIGREE OF MOLLUSKS

					Dibranchia
		Heteropoda			│
		│	Prosobranchia		│
Pulmonata │ │	Lipobranchia │ │	│ │ │	│ │ │		Tetrabranchia Cephalopoda (Cuttles or Poulps)
│	Gymnobranchia	│	│		│
│	│	│	│		│
│ │	Pleurobranchia Opisthobranchia	│		Chitonides	│ │
│	│	│		│	│
					│
		Delocephala			│
		│			│

		Inclusa		│	Pteropoda
		│		│	│
		│
		Sinupalliata		│
		│	Scaphopoda	│
		│	│	│
	Sclerobrachia	│ Integripalliata Siphoniata │	Perocephala Cochlides (Snails)
	│	│	│
	Sarcobrachia Testicardines	Asiphonia Lamellibranchia (Mussels)	│ │ │
	│	│	│
	│
	Ecardines Spirobranchia (Lamp-shells)	│ Otocardia (Molluscs with chamber and ante-chamber to the heart)
	│	│

Promollusca (Primæval Molluscs) Molluscs with simple heart
│
(Worms)
│
Gastræa

We distinguish two sub-classes among the Snails (Cochlides, or Gasteropoda), namely, the Stump-headed and the Large-headed Snails. The Stump-headed Snails (Perocephala) are very closely allied to Mussels (through the Tooth-shells), and also to the Cuttle-fish (through the Butterfly-snails). The more highly developed Snails, with large heads (Delocephala), can be divided into Snails with gills (Branchiata) and Snails with lungs (Pulmonata). Among the latter are the Land-snails, the only Molluscs which have left the water and become habituated to a life on land. The great majority of Snails live in the sea, only a few live in fresh water. Some River-snails in the tropics (the Ampullaria) are amphibious, living sometimes on land, sometimes in water, and at one time they breathe through gills, at another through lungs. They have both kinds of respiratory organs, like the Mud-fish and Gilled Newts among the Vertebrata.