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As regards the remaining organs of the Amphioxus, we need only mention that the gonads or sexual glands are developed very late, immediately out of the inner cell-layer of the body-cavity. For the rest, the subsequent development into the adult Amphioxus of the larva we have followed is so simple that we need not go further into it here.
We can say, for instance, that we have inherited the oldest organs of the body, the external skin and the internal coat of the alimentary system, from the Gastraeads; the nervous and muscular systems from the Platodes; the vascular system, the body-cavity, and the blood from the Vermalia; the chorda and the branchial gut from the Prochordonia; the articulation of the body from the Acrania; the primitive skull and the higher sense-organs from the Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot from the Amphibia; the palate from the Reptiles; the hairy coat, the mammary glands, and the external sexual organs from the Pro-mammals.
Of the other chief organs we have still to mention the sexual glands, which lie right behind in the body-cavity. All the Ascidiae are hermaphrodites. Each individual has a male and a female gland, and so is able to fertilise itself. Fertilisation is accomplished here, and in many of the Ascidiae developed embryos are found.
In the struggle for life they have found shelter in the body-cavity of other animals. Modern gastraeads. In order to avoid confusion with these, I afterwards gave them the name of Prophysema. The whole mature body of the Prophysema is a simple cylindrical or oval tube, with a two-layered wall. The two strata of cells that form the wall of the tube are the primary germinal layers.
But as soon as the cement-ducts penetrated into the body-cavity of the host and were bathed by its fluids, an endosmotic interchange must necessarily have been set up between the materials dissolved in these fluids and in the contents of the cement-ducts, and this interchange could not be without influence upon the nourishment of the parasite.
Coelenteria, without anus and body-cavity. : 6. Gastraeades. With two germ-layers. : 6. Gastrula. Hydra, Olynthus, Gastremaria. 6 to 8. Coelenteria, without anus and body-cavity. : 7. Cryptocoela. Convoluta, Proporus. 6 to 8. Coelenteria, without anus and body-cavity. : 8. Rhabdocoela. Vortex, Monotus. 9 to 11. Vermalia, with anus and body-cavity. : 9. Provermalia. Gastrotricha.
At first the chorda only consists of a single row of large entodermic cells. It is afterwards composed of several rows of cells. In the Ascidia-larva, also, the chorda develops from the dorsal middle part of the primitive gut, while the two coelom-pouches detach themselves from it on both sides. The simple body-cavity is formed by the coalescence of the two.
It seems to be true that blood-cells may be formed from the cells of the entoderm before the development of the mesoderm. In this we see clearly how a number of stellate cells proceed from the "vascular layer" and spread in all directions in the "primary body-cavity" i.e. in the spaces between the germinal layers.
As these regular rythmical pulsations recur the blood is forced forward through the heart into the head where it bathes the organs there. We shall see in another chapter that the malarial parasite escapes from the walls of the stomach of the mosquito into the blood in the body-cavity and finally reaches the salivary glands.
Our lungs, trachea, and larynx are formed from the ventral wall of the branchial gut. These vesicles are found in all the Vertebrates except the two lowest classes, the Acrania and Cyclostomes. In the lower Vertebrates they do not develop into lungs, but into a large air-filled bladder, which occupies a good deal of the body-cavity and has a quite different purport.
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