Structure and Development of the Nephridia. 361 



The limits of the nephrostome as such are thus sharply clefiucd 

 iu the adult. In early stag-es, on the other hand, no sharp line of 

 demai'cation can be drawn. The above difference of structure must 

 not therefore, as some suppose, be held to indicate that the two 

 regions represent geuetically distinet structures which have secon- 

 darily entered into intimate association. It will be shown beyond 

 that the two portions develop iu direct continuity with one another 

 as portions of an orig-inally single embryouic rudiment. 



The dorsal lip is bordered by a series of bright red vascular 

 ciliated processes. These are flattened and somewhat leaf-shaped 

 structures, partìally subdivided by secondary finger-like processes, 

 aud arranged in a single row from end to end of the dorsal lip. 

 The dorsal lip is somewhat the longer of the two, and is usually 

 somewhat folded in the living Arenicola^ so that the oue-rowed 

 arraugement is not obvious on casual examination. When the 

 nephrostome is slightly stretched , however (as in Fig. 2) , the pro- 

 cesses are readily seen to have the above-described arraugement, 

 with their broad faces opposed like the leaves of a hook. At the 

 anterior and posterior limits of the lip they diminish in size and 

 the number of subdivisions becomes less. Each process may be 

 described as palmate in shape; at its somewhat uarrowed base it 

 is attached to the border of the dorsal lip; in structure it is essen- 

 tially a hollow thin-walled flattened vesicle, lined iuternally by a 

 eounective tissue basement membrane and covered by a columnar 

 ciliated epithelium which passes without transition into the epithe- 

 lium liuing the interior of the nephrostome. 



The relation of the processes to the nephrostomial vessel is 

 in reality very simple, and may be readily understood by a reference 

 to the textfigure (p. 362), which represents in a somewhat simplified 

 form a cross section of the entire anterior region of the nephridium. 

 The main portion of the vessel [nst.v.) runs along the outer margiu 

 of the dorsal lip immediately below the Insertion of the processes, 

 and its interior communicates directly with that of the processes, 

 whose bright red color is thus due to the contained blood. No 

 distinction, however, can be seen between the wall of the blood- 

 vessel and the internai connective tissue lining or basement-mem- 

 braue of the processes, the two being directly continuous with one 

 another, aud in reality dififeriug only in their relations to the nephro- 

 stomial epithelium. This epithelium is intimately related to that 

 part of the blood-vessel which enters the interior of the processes. 



24* 



