ANATOMY OF CEPHALODISCUS NIGRESCENS. 



35 



is a triangular space occupied by the median septum between the right and left collar 

 cavities ; and along each face of the septum there run muscle fibres in the direction 

 indicated in text-fig. 10 at mu. la. (See also text-fig. 12, mu. la.) Between the collar 

 septum and the central nerve mass is a narrow cleft, the dorsal blood sinus ; this is 

 marked (/. h. s. in text-fig. 12, and is shown, but not marked, in text-fig. 10. There 

 seems to be no direct connection between this space and the cavity of the heart. On 

 the ventral side of the middle part of the notochoi'd is an irregular tissue (text-fig. 10), 

 but there is no cavity that can be identified as the ventral blood sinus of Masterman. 



The heart (text-figs. 10, 11 and 12, h) is a sac, apparently closed, situated with 

 its front part free in the pericardium and the hinder part attached to the lower 

 surface of the notochord. Projecting from the surface of the heart are irregular, 

 short threads, which appear to be broken coelomic trabeculae. They have small 

 spherical nuclei placed upon their sides, as one finds in the coelomic trabeculae of 

 the proboscis cavity and collar cavity, and they probably extended, when perfect, to 

 the wall of the pericardial sac. 



Text-Figube 11. — Section of a polypide of Cephalodiscus nigrescens taken transversely to the length of the body, and 

 passing through the proboscis pores and the front part of the heart. The odd sections of plumes which are almost 

 invariably included in such a preparation as this are not shown. 



b.s. = thickened ventral wall of the buccal shield ; c.n.m. = central nerve mass ; /;. = heart ; mic.S = radiating 

 muscle fibres passing across the cavity of the buccal shield; n.t.l, n.t.2 = nerve tracts (see text) ; p.c. = cavity of 

 the buccal shield, proboscis cavity ; 114}. = external opening of the buccal shield, proboscis pore. 



The buccal shield has frequently a strong transverse wrinkle such as is shown 

 in text-fig. 10, in front of the red-line (r. /.) The posterior lobe of the shield is 

 thinner than the central part, and consists of the ventral and dorsal or ad-oral 

 walls almost in contact, the coelomic space being here reduced to a barely 

 recognisable cleft. 



The proboscis pores, or openings of the buccal shield, are a pair of narrow 

 tubular passages lined with prismatic epithelium, and situated at the sides of the 

 pericardium (text-fig. II, p. ]).) and at a level anterior to the tip of the notochord. 

 The nerve tracts marked c. 11. m. and n. t. I in text-fig. 11 are continuous both in 

 front and behind the proboscis pores. The ectodermal pit (text-fig. 10, e. p.) is 

 constant in its occurrence, l)ut it has the form of a wrinkle in the antero-dorsal 

 wall of the shield rather than of a definite sensory organ, a relation which bears 



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