THE BNTBROPNBUSTA. 45 



As the "dorsal roots" do not occur in B. Kowalevskii 

 their development has not, unfortunately, been observed. 



The Hypoblastic Structures. 



The notochord has been described already, as also the 

 mode by which the mouth comes to be anteriorly directed. 



The cavity into which the mouth leads is lined by very thick 

 walls (figs. 90j 67, &c.), composed of long cells supported by 

 some intracellular substance, probably the same as that of the 

 notochord. In B. Kowalevskii it leads continuously into 

 the branchial chamber, but in the other species, in which the 

 branchial chamber is separated by longitudinal ridges (fig. 91), 

 from the lower cavity of the branchial region (which thus has 

 the well-known figure-of-8 shaped cavity). The anterior end 

 of the branchial cavity comes to be almost enclosed in the 

 pharyngeal cavity. As the result of this on either side the 

 branchial cavity projects as two blind horns, which are en- 

 closed in the pharyngeal cavity. 



The structure of the gill-slits has been sufi&ciently described 

 by Kowalevsky, Agassiz, and Spengel. 



To these accounts there is little to add. The figures 84 and 

 85 illustrate the mode by which their final structure is 

 attained. It is practically impossible to follow their structure 

 by means of transverse sections, but longitudinal sections and 

 surface-views make them easily intelligible. Each gill-slit of 

 B. Kowalevskii is U-shaped and surrounded by- a skeletal 

 secreted structure, as shown in fig. 85. In my last paper I 

 stated that, though the origin of these structures was uncertain, 

 the balance of evidence favoured the view that they were hypo- 

 blastic. Since the above was written I have been led to 

 regard them as more probably mesoblastic, owing to some of 

 the appearances since observed. It should be noticed that the 

 body cavity is continued into the valves always, but never into 

 the bars separating adjacent gill-slits in which the bordering 

 bars are in contact. This is due to obliteration of the cavity 

 by the skeletal bars. This feature is very useful in distin- 

 guishing these parts in sections. 



