We can distinguish (I) digestive endoderm, in the stomach, often with special glandular elements; (2) circu-, latory endoderm, in the radial and ring canals; (3) supporting endoderm in the axes of the tentacles and in the endodermlamella; the latter is primitively a double layer of cells, produced by concrescence OC-- = w.?"
In the wall of the sack is a double layer of endoderm, the space between which is a continuation of the coelenteron.
I also cover the top-growth of susceptible plants and those of borderline hardiness with a double-layer of garden fleece.
Finally, a thin double layer of smooth muscle is often present - the muscularis mucosa for local movement of the mucosa.
A wing is an outgrowth from the dorsal and pleural regions of the thoracic segment that bears it, and microscopic examination shows it to consist of a double layer of cuticularized skin, the two layers being in contact except where they are thickened and folded to form the firm tubular nervures, which serve as a supporting framework for the wing membrane, enclose air-tubes, and convey blood.