ventrally open into the first somite of the trunk. The mesoblastic masses of the head are probably not to be regarded as forming a pair of somites equivalent to those in the trunk, but as forming the mesoblastic part of the præ-oral lobe, of which so much has been said in the preceding pages. Kleinenberg's observations are however of great importance as shewing that the cephalic cavities are simply an anterior part of the true body cavity. The splanchnic layer of the head cavity gives rise to the musculature of the oesophagus. The somatic layer of the trunk somites becomes converted into the musculature of the body wall and the external peritoneal layer of body cavity. The first part of the muscular system to be definitely formed is the ventral band of longitudinal muscles which arises on each side of the nervous system in contact with the epidermis (fig. 157, _m_). How the circular muscles become subsequently formed outside these muscles has not been made out. The splanchnic layer of the trunk somites gives rise to the muscular and connective-tissue wall of the mesenteron, and also to the walls of the vascular trunks. The ventral vessel is first formed (Kowalevsky) as a solid mass of cells which subsequently becomes hollowed out. The dorsal vessel in Lumbricus and Criodrilus is stated by Kowalevsky and Vejdovsky to be formed by the coalescence of two lateral vessels; a peculiarity which is probably to be explained by the late extension of the mesoblast into the dorsal region. The layer from which the sacks for the setæ and the segmental organs spring is still doubtful. The sacks for the setæ are believed by Kowalevsky (No. 342) to be epiblastic invaginations, but are stated by Hatschek (No. 339) to be mesoblastic products. For the development of the segmental organs the reader is referred to the chapter on the excretory system. In marine Polychæta the generative organs are no doubt mesoblastic products, as they usually spring from the peritoneal epithelium, especially the parts of it covering the vascular trunks. The Alimentary Canal. In Lumbricus the enteric cavity is formed during the gastrula stage. In Criodrilus the hypoblast has at first no lumen, but this becomes very soon established. In Euaxes on the other hand, where there is a true epibolic gastrula, the mesenteron is at first represented by a solid mass of yolk (_i.e._ hypoblast) cells. As the central amongst these become absorbed a cavity is formed. The protoplasm of the yolk cells which line this cavity unites into a continuous polynuclear layer containing at intervals masses of yolk. These masses become gradually absorbed, and the protoplasmic wall of the mesenteron then breaks up into a cylindrical glandular epithelium similar to that of the other types.