Structure and Development of the Nepliridia. 379 



defined so soon as the ventral vessel, heing possibly less necessary 

 iu early stag-es thaii tbe latter, iu wliich the blood flow is from 

 betöre backward towards tbc growiug zone. This vessel, as will 

 be Seen from Fig-. 9, Piate 22, early appears as a well delined 

 Channel extending almost to tbe posterior extremity; it arises as a 

 space between tbe intestinal wall and tbe mesoderm and soon be- 

 comes enclosed on all sides by tbe extension of tbe mesoderm cells. 

 Its walls are from tbe first continuous witb tbe original septa, a fact 

 of significance, since tbe segmental blood-vessels arise in tbe septa 

 and in tbis manner acquire tbeir cbaracteristic relations to tbe 

 uepbridia, as will be explained beyoud. 



Mesoderm cells remain applied to tbe wall of tbis vessel, and 

 as development proceeds they acquire a voluminons and vacuolated 

 appearance, and pigment or excretory granules become deposited in 

 tbeir interior. Tbese cells constitnte tbe chloragog-en cells; tbey 

 are tbus in reality modified peritoneal cells, wbich througbont life 

 coat the ventral blood-vessel and play an important part in tbe 

 process of excretion (cf. Eosa 1903). Tbey bave in most larvae 

 already assumed tbeir excretory character in tbe third somite in 

 front of tbe growing zone, and from this region tbey may be seen 

 to pass backwards by a graduai trausitiou into tbe posterior un- 

 difi'ereutiated meseuchyme cells (Fig. 9). Tbeir characteristics will be 

 cousidered in greater detail below. 



Dorsal and ventral blood-vessels soon acquire open communica- 

 tion witb eacb otber in tbe growing zone, thus completino- tbe circu- 

 latiou and providing more fuUy for tbe nutrition of this region. 

 Tbe nature of tbis commuuication may be seen from Fig. 24, Piate 23, 

 wbicb is a cross section tbrougb tbe posterior region of a larva of 

 11 — 12 somites, passing tbrougb tbe region of tbe primitive septum 

 between somites XI and XII. Dorsal and ventral vessels are bere 

 seen to be connected by ablood-sinus wbich completely surrounds 

 tbe intestine. A sinus of this nature is typically present in larvae 

 of this and later stages; it consists of a simple space between the 

 intestinal wall and the mesoderm, and is situated usually at the 

 insertion of the most posterior primitive septum, but frequently (see 

 Piate 23, Fig. 30 and Piate 24, Fig. 32) at tbe membrane wbich separates 

 the posterior mesoderm from the space between proctodaeum and 

 ectoderm in tbe post-mesodermal region. The sinus varies greatly 

 in its size and in the amount of blood that it contains and at times 

 seems to become obliterated. Apparently it is formed as required 



