392 R'''pl> S. Lillie 



continuoiis. As the epithelium increases in extent the vessel acquires 

 its definitive position just external to the margin of the dorsal lip. 



In this way are defined the limits of the future dorsal and ventral 

 lips, the former of which is distinguished by the presence of the 

 blood-vessel which runs along its entire margin. At the inner 

 (future posterior) angle of the nephrostome the vessel passes on to the 

 body of the nephridium and gradually extends backwards to form 

 the nephridial vessel. This vessel also represents a space between 

 the Peritoneum and the nephridial epithelium, and is formed as a 

 simple coutinuation of the nephrostomial vessel. A slight peritoneal 

 fold extending along the length of the nephridium (Piate 23, Figs. 25, 

 26) marks in early stages the position of the future vessel. In the 

 more advanced nephridium represented in Fig. 27 the vessel has 

 become well-defined iu the anterior part of the nephridium, while 

 posteriorly it is still represented by the peritoneal fold. Later the 

 vessel completes its backward extension and forms, as well known, 

 the chief vascular supply of the glandulär region and terminal 

 vesicle. 



It is impossible in this place to give a full account of the manner 

 in which all the blood-vessels of Arenicola arise, and the above 

 brief account serves simply to explain the manner in which the 

 characteristic relations arise between the segmental blood-vessel and 

 the nephrostome. We have seen that the nephrostomial blood-vessel 

 is formed as a portion of the system of septal blood-vessels; and 

 that its association with the funnel depends simply upon the fact 

 that this structure itself represents in large part simply a differentiated 

 part of the septum*. 



Shorth^ after the appearance of the system of septal blood-ves- 

 sels the membrauous portion of the septum disappears^. The 

 nephrostome then undergoes a change of position, its dorsal border 

 assuming the antcro-posterior direction of the nephridial blood-ves- 

 sel, and the aperture consequently becoming directed inwards 



* For tlie vascular supply of the adult nephridium see Gamule & Ashworth, 

 1900. Their Fig. 29 shows iu a simple and clear manner the essential vascular 

 relations of the nephridia. The detailed distribution of the blood-vessels of 

 A. cristata has been the subject of investigation at the Marine Biological 

 Lal)oratory, the results of which are not yet published. 



2 Witli the exception of a small portion extending from the anterior angle 

 of the nephrostome to the body-wall and hearing the extra-nephridial part of 

 the segmental vessel (see Piate 22, Fig. 6a. cf. also Gamble & Ashworth 1900, 

 Figs. 53—54). 



