On the Structure of the Gut of the Ascoglossan Nudibranchs.



As regards anatomy and histology the alimentary canals of Elysia viridis, Limapontia capitata and Hermæa dendritica are very similar and exhibit specializations for manipulating their filamentous algal food. The mouth leads through an oral tube to a small vestibule ventral to the anterior tip of the odontophore. Into the vestibule and the tube open subepithelial glands which secrete mucus and adhesive substances. The vestibule leads into the dorsal part of the buccal cavity containing the odontophore. This is lined by a cuticularized epithelium and muscles—-striped in Hermæα—radiate from the basement membrane or from the cuticle. The buccal cavity with its thick muscular walls acts as an organ for sucking into the alimentary canal the contents of plant-cells. The salivary glands open by ducts into its posterior part, behind the opening each duct swelling into a muscular vesicle which acts as a pump. The saliva probably contains enzymes.

The œsophagus, ciliated and glandular, passes to the stomach. In Elysia and Herma a muscular œsophageal pouch is developed which assists in the transport of food along the gut.

The small stomach, of little physiological significance, receives the wide ducts of the digestive gland. In Elysia these open into the left and mid-posterior walls. In Limapontia and Hermæa the posterior part of the stomach opens freely into the main tubules of the gland, which are sufficiently wide to accommodate sudden inflows of food pumped from the œsophagus. Owing to the type of food, from which the fæcal matter is inconsiderable, and the total absence of a mantle-cavity, there is no necessity for extensive arrangements to elaborate the fæccess; consequently the intestine is short and, except in Hermæa, without glands.

The gut and the nervous system show greater resemblances to those of the Lophocercidæ and Diaphanidcæ than to those of the other groups of nudibranchs. It is therefore in the primitive groups of the tectibranchs that the origin of the Ascoglossa is to be sought.