Internally the muscular layers are lined by an epithelium.
For pharmaceutical purposes crude petroleum is no longer generally used by civilized races, though the product vaseline is largely employed in this way, and emulsions of petroleum have been administered internally in various pectoral complaints; while the volatile product termed rhigolene has been largely used as a local anaesthetic.
When salicin is taken internally no irritant action occurs, nor is there any antisepsis.
The Heteronemertini thus appear to have developed an extra layer of longitudinal fibres internally to those which they inherited from more primitive ancestors, whereas the Metanemertini are no longer in possession of the internal circular layer, but have on the contrary largely developed the external circular one, which has dwindled away in the Heteronemertini.
This telson may enlarge, it may possibly even become internally and sternally developed as partially separate somites, and the tergum may remain without trace of somite formation, or, as appears to be the case in Limulus, the telson gives rise to a few well-marked somites (mesosoma and two others) and then enlarges without further trace of segmentation, whilst the chitinous integument which develops in increasing thickness on the terga as growth advances welds together the unsegmented telson and the somites in front of it, which were previ ously marked by separate tergal thickenings.