THE ENTEEOPNEUSTA. 49 



1. Ectoderm.— Ciliated tailed cells. 



Glandular cells. 

 Nerve-fibres as a layer. 

 Basement membrane. 



2. Narrow tissue space crossed by ingoing fibres from ecto- 

 derm, and by supporting fibres in all directions, together with 

 a very few circular fibres {v. fig. 51). 



3. Tract densely filled with radial and longitudinal muscles 

 (in B. Kovs^alevskii concentrically disposed in rings) and 

 connective tissue. 



4. The tissue space into which the central organs project. 



5. The central organs : 



(a) Proboscis gland with its sac. 



(b) Heart. 



(c) Notochord. 



The muscles of the collar body cavity in B. Kowalevskii 

 are not gathered into bundles or definitely arranged, excepting 

 those which are attached to the lateral rods of the axial skeleton 

 (fig. 60) . These large muscles are inserted into the back oj 

 the collar. The whole cavity between the pharynx and the 

 skin, being originally second pair of mesoblastic pouches, be- 

 comes obliterated, being filled with muscles and connective tissue. 



In B. salmoneus, B. Robinii, and B. Brooksii this also 

 occurs, but in B. salmoneus (fig. 106) the longitudinal 

 muscles are grouped into bundles. These bundles form two 

 series, the one on the somatic and the other on the splanchnic 

 side, and in the narrower parts of the cavity the groups of the 

 two series dovetail into each other (fig. 106), being each 

 gathered around a connective tissue septum projecting into the 

 cavity. 



These fibres in B. Robinii occasionally, after osmic acid, 

 show a slight striping (fig. 94, c). 



In B. minutus the longitudinal muscles of the collar lie in 

 a layer immediately under the skin and under the pharyngeal 

 wall. The cavity is crossed by many radial fibres, upon which 

 some cells are placed, but is not so much filled up as in the 

 other species. 



