ON THE STRUCTURE OF VEEMICULUS PILOSUS. 26] 



of varying ages the development of these organs can be traced, 

 and the result has been diagrammatically represented in fig. 

 13. The spermatozoa are shed at an early stage of develop- 

 ment into segment 10, and the anterior septum of this 

 segment soon bulges out, forming a sac — the anterior sperm- 

 sac (fig. 13, a. sp. s.). Later on this sperm-sac pushes its 

 way across segment 9, through its anterior septum into seg- 

 ment 8. The hinder wall of segment 10 also bulges out, 

 forming the posterior sperm-sac (fig. 13, p. sp. s.) The ova 

 are shed into segment 11. The hinder septum of this segment 

 forms the ovisac (fig. 13, ov. s.), which ultimately pierces as 

 many as seven or eight septa behind it.^ The posterior 

 sperm-sac also enlarges, enters the ovisac, and grows back 

 through five or six segments. It is within the space between 

 the wall of the ovisac on the outside and the wall of the 

 posterior sperm-sac within, that the blood-vessels run which 

 supply nourishment to the ova (fig. 5, v. ov.) ; it is also, no 

 doubt, through this space that the ova escape back again to 

 the 11th segment when ripe. 



The clitellum extends over segments 10 — 12, and part of 

 segment 13. 



Summary and Conclusions. 



The chief characters ofVermiculus pilosus are therefore 

 the following : 



Four bundles to each segment of furcate setse, generally 

 three per bundle. 



A dense covering of hair-like processes. 



A vascular system, containing red blood, and composed of a 

 dorsal and a ventral longitudinal vessel communicating by 

 means of lateral vessels which branch on to the body-wall. 

 The absence of hearts or commissural vessels. An elaborate 

 system of unicellular valves in the longitudinal and transverse 

 dorsal vessels. 



> The ovisac pierces the septa at a point nearer to the dorsal blood-vessel 

 than is represented in the diagram. 



VOL. 37, PART 3. NEW SER. S 



