524 ARTHUR DENDY. 



tion, and as the wall of the oscular tube, at any rate in its 

 upper portion, seems to me to have as much claim to be re- 

 garded as ectosome as it has to be regarded as choanosome, 

 I shall describe it in this place. The osculum (fig. 9, o.) leads 

 direct into a widely- expanded canal, the oscular tube, about 

 0'8 mm. in diameter. The walls of this canal present a series 

 of well-defined circular ridges of fibrous tissue. Owing to the 

 fact that the oscular tube does not run parallel with the plane 

 of section, the section figured shows these fibrous ridges 

 (fig. 9,/. r.) in transverse section in the upper portion of the 

 tube, and in vertical longitudinal section in the lower portion. 

 The ridges themselves do not necessarily run parallel with one 

 another but may branch and anastomose. Each consists, for 

 the most part, of a dense band of slender fibres (fig. 12) 

 running round the oscular tube, and showing here and there 

 elongated granular bodies, which I believe to be nuclei. The 

 fibres themselves exhibit a very distinct, wavy outline, strongly 

 calling to mind the appearance of ordinary white fibrous con- 

 nective tissue. This fibrous band is covered ou the outside by a 

 layer of curious, granular, flocculent-looking tissue, the nature 

 of which I do not at present understand (fig. 12,/.). I have 

 little doubt that these rings of fibrous tissue around the oscular 

 tube act as sphincter muscles, whereby the diameter of the 

 tube is regulated. The concentration of the fibrous tissue in 

 distinct annular bands is probably to be regarded as indicating 

 a higher degree of differentiation than that which occurs in 

 Ridleia, where the fibrous tissue forms a continuous sheath. 



The Choanosome. — Owing to the lacunar character of the 

 canal system the choanosome is much less dense and compact 

 than in Ridleia. The amount of mesodermal tissue in pro- 

 portion to the rest of the choanosome is also greater, and it 

 exhibits numerous deeply staining, irregularly shaped cells 

 (probably the amoeboid cells) embedded in a coarsely granular 

 and at the same time somewhat gelatinous-looking matrix 

 (fig. 10, m. c). 



The Canal System. — We owe by far the greater portion 

 of our knowledge of the arrangement of the canal system in 



