378 L. DONOASTER. 



In the third-day larva it is seen at once that from the head 

 as far as the genital cells the septum dividing the two halves 

 of the mesoderm is much thicker in the middle, though 

 dorsally and ventrally it narrows to a thin mesentery. It 

 also contains numerous nuclei instead of the small number 

 that were scattered at intervals up to this time. In the 

 mesoderm the muscles are now well advanced, and appear as 

 three pinnate g'roups {imis.) in each quadrant, especially in 

 the tail region, where they are relatively larger (fig. 32). 

 Over each group lies a nucleus, which is either that of the 

 muscle-cell or belongs to a very slender peritoneal epithelium, 

 but from sections of the adult I am inclined to doubt the 

 existence of the latter. 



As the nature of the muscles and coelomic wall is of great 

 importance in fixing the systematic position of the Chteto- 

 gnatha, it will be well at this point to review the whole 

 evidence. It is seen in figs. 13, 14 (PI. 19) that the nuclei of 

 the somatic mesoderm, although collected into four groups at 

 the outer sides of the mesodermal strands, do not lie actually 

 in contact with the junction between the mesoderm and ecto- 

 derm. The nuclei in each group lie on the inner face of a 

 mass of tissue, which stains more deeply than ordinary cell 

 protoplasm, and fig. 30 (PI. 21) shows that this tissue is the 

 rudiment of the muscles. It is found, however, that it never 

 contains nuclei, while the nuclei which appear on the surface 

 of each mass seem to belong to the cells which compose it, 

 lying at their inner ends. As the somatic mesoderm is 

 traced through the later stages the same condition is found ; 

 in the first and second day after hatching the only difference 

 is that the tissue beneath the nuclei is enlarging and 

 becoming fibrous, while on the third and subsequent days the 

 muscles, with their pinnate fibres, arc fully formed. 



When a section of an adult Sagitta is examined (cf. fig. 36) 

 the muscles have increased largely in number, but otherwise 

 no alteration is found ; the nuclei still appear at the inner 

 ends of the pinnate groups of fibres. In some cases, e. g. in 

 the series from which figs. 30, 37 were drawn, the muscles 



