The Ectoderm.

The ectoderm, pharynx and peripharyngeal space are covered by a thin transparent cuticle. The ectoderm of the body is unusual in that the dorsal ectoderm is composed of thicker cells than those of the ventral ectoderm, and these dorsal cells do not have cilia. They are columnar cells. The tissue also contains rhabdites and globular glands that secrete mucus. The ventral ectoderm is more typical in that it is ciliated. These cells are cubical; and certain of them, arranged in bands converging on the pharyngeal opening, bear longer cilia.

Histologically a typical ectoderm cell has a round central nucleus in which no nuicleolus but several chromidia may be distinguished. The cytoplasm is generally clear around the nucleus, but elsewhere it contains small granules and fatty drops. The base of the cell is broad, and connected to it are fibrillae. The basal granules are difficult to distinguish without a special technique. All cells seem to possess considerable powers of expansion, especially laterally.

The rhabdites are short and thick when undischarged, but, when discharged, are long and slender with a rounded distal knob. At the base of each is a coiled secretion apparently of the same composition as the rhabdite itself. The rhabdite is secreted, as is usual with these animals, by a special cell deep in the mesoderm.

Stripe pigment can be seen in section as a heavy deposit of dark brown granules in groups of special parenchyma cells lying about the region of the circular muscles. The ground colour pigment is deposited in a thick band of small parenchyma cells under the storage tissue. There is some indication that the pigment deposits alter in depth in the tissues under different conditions.

The ectoderm of the pharynx is not ciliated, nor is the ectoderm lining of the peripharyngeal space, otherwise this ectoderm is quite similar to that of the ventral ectoderm.