36 W. G. RIDEWOOD. 



out the conclusion arrived at by Harmer in the case of C. dodecalophus that the 

 importance attributed to the ectodermal pit by Masterman is unmerited. 



From two small areas on the dorsal wall of the buccal shield there radiate 

 muscle fibres which are inserted into the coelomic face of the thickened central 

 part of the ventral wall of the shield (text-fig. 11, mu. 3 ; see also text-fig. 12). 



The muscles that run between the basement membrane underlying the central 

 nerve mass and the notochord, on either side of the median septum between the 

 right and left collar cavities (text-figs. 10 and 12, mu. la), send some of their fibres 

 backward on the right and left sides of the mouth, just ventral to the dorsal 

 diverticulum. Posterior to the level of the oral aperture these converge, and 

 become applied in the form of two sheets to the sides of the septum which divides 

 the right and left collar cavities in the region of the posterior edge of the post-oral 

 lamella (text-fig. 10, mu. Ic), and to adjacent parts of the dorsal and ventral walls 

 of the collar cavity. The two tracts of muscle shown in text-fig. 10 at mu. la. and 

 mu. Ic. are thus in reality terminal parts of the same muscle. The middle part 

 runs by the side of the mouth, and is seen in text-fig. 13 at mu. lb. This muscle, 

 called by Harmer the " oral muscle," stops immediately in front of the anterior 

 end of the muscle tract of the ventral body wall (text-fig. 10, mu. 2), which 

 continues back into the stolon as part of the longitudinal muscle of that organ, 

 but, as Harmer has pointed out in the case of the species examined by him, the 

 muscle fibres do not pass through the septum that separates the collar cavity from 

 the trunk cavity. 



At the side of the base of the notochord a few of the fibres pass ventrally and 

 terminate in the septum between the collar cavity and the proboscis cavity on the 

 dorsal side of that small area from the ventral side of which the radiating muscles 

 of the proboscis cavity arise. Beneath the central nerve mass a few of the fibres of 

 mu. la are continued forward into the dorsal wall of the front part of the shield, 

 passing externally to the proboscis pores; others pass out laterally as a thin 

 sheet lying immediately below the nerve mass, and these are continued into the 

 plume-axes. 



The section drawn in text-fig. 12 is taken only slightly behind that shown in 

 text-fig. 11. It shows the front parts of the right and left collar cavities, separated 

 from one another by a muscle-flanked septum {mu. la), and separated by more 

 extensive septa from the cavity of the shield. The base of each half of the lophophore 

 divides roughly into three parts before breaking up into the seven plumes ; the base 

 is necessarily cut obliquely, and the outline is consequently irregular. The cavity of 

 the collar is continuous with that of the lophophore and its plumes ; the indicating line 

 marked h.L, points to the cavity of the lophophoral base. 



The central nerve mass {c.n.m.) shows a few large ganglion cells in its dorsal 

 layer immediately beneath the superficial epithelium, and the nerve tracts that lead 

 out sideways into the base of the lophophore are continued into the plumes as the 



