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In Prophysema the primitive gut is a simple oval cavity, but in the closely related Gastrophysema it is divided into two chambers by a transverse constriction; the hind and smaller chamber above furnishes the sexual products, the anterior one being for digestion. The only material difference between them is that in the sponge the thin two-layered body-wall is pierced by numbers of pores.
Moreover, we have seen that the parapodia of annelids naturally point to the development of an external skeleton, for their muscles are already a part of the external body-wall and attached to the already existing horny cuticle. The logical goal of their development was the insect.
The gonads are among the oldest organs, the few other organs that we find in the Platodes between the gut-wall and body-wall being later evolutionary products. They often have a number of branches. They are first met in the Turbellaria, and have been transmitted direct from these to the Vermalia, and from these to the higher stems.
At each side of the branchial gut there are between twenty and thirty roundish four-cornered sacs, which can clearly be seen from without with the naked eye, as they shine through the thin transparent body-wall. These sacs are the sexual glands they are the same size and shape in both sexes, only differing in contents. Both sacs lie on the inner wall of the atrium, and have no special outlets.
The organism itself consists of a minute particle of protoplasm, a single cell with no definite shape or body-wall and no specialized organs or apparatus for carrying on the life-functions.
They are often quite large, a foot or even more in length. They are composed of many, often several hundred, rings or segments. Between these the body-wall is thin, so that the segments move easily upon each other, and thus the animal can creep or writhe. These segments are very much alike except the first two and the last.
Another suggestion worth considering is that the whale completely empties the gas out of its lungs by muscular compression of the body-wall before diving, so that there is no gas left in the body to be acted on by the increased pressure resulting from its sinking into deep water.
In the turbellaria we find for the first time a true body-wall distinct from underlying organs. The outer layer of this is a ciliated epithelium or layer of cells. Under this an elastic membrane may occur. Then come true body muscles, running transversely, longitudinally and dorso-ventrally.
His reason for this distinction is the fact that in the latter the organs are inclosed within walls of their own, distinct from the body-wall; whereas in the former the organs are formed by internal folds of the outer wall of the body, as in the Polyps, or are hollowed out of the substance of the body, as in Jelly-Fishes.
These important early embryonic processes take place so quickly in the Amphioxus that four or five hours after fecundation, or about midnight, the spherical blastula is completed. The wall is at once gut-wall and body-wall. It is composed of two simple cell-layers, the familiar primary germinal layers.
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