6 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



into close relation with the lateral blood vessels or lacunar diver- 

 ticula of same, but with which they have no actual comuninica- 

 tion. One or more erterent ducts usually lead from each nephridial 

 canal to the exterior of the body. Most commonly the nephridia 

 are limited to a comparatively short area in the esophageal region, 

 but in some forms (Stichostemma) extend the whole length of the 

 body. In most species of Hoplonemertea the nephridiopores are 

 situated ventral to the lateral margins of the bodv, \\ hile in most 

 species of the other groups they lie on the dorso-latcral asjtects of 

 the body. 



Nervous system. — The central nervous system is made up princi- 

 pally of a four-lobed brain, the two lobes of each side being closely 

 united with each other and joined to those of the other side by a 

 commissure above and one below the rhynchodaeum, and a pair of 

 large lateral nerves, accompanied by ganglion cells, extending fi-oni 

 the ventral pair of brain lobes to the posterior end of the body. 

 The lateral nerves are connected at intervals by a nervous plexus, 

 and at the posterior end of the body by a distinct commissure 

 usually situated dorsally to the anus. In addition, a dorso-inedian 

 nerve is commonly present, and sometimes a ventro-median one; 

 most forms have a pair of well-developed esophageal nerves, and 

 nearly all have special proboscis nerves, together with peripheral 

 nerves to the integument, ocelli and other sense organs. 



flense organs. — A pair of specialized cerebi-al sense oi'gans is 

 often present as posterior appendages of the dorsal brain lobes 

 (Heteronemertea), or they may lie anterior or beside the brain in the 

 other orders ; in only a few forms are they wanting. Other special 

 sense organs are found on the lateral margins of the bodv in manv 

 of the Paleonemertea. Ocelli are often present in great numbers, 

 but are wanting in the Paleonemertea and in many species of the 

 Heteronemertea. A few species possess otoliths; some have a 

 specialized frontal sense organ, while several species of Carinoma 

 possess a number of sensory pits on the dorsal aspect of the head. 



Reproductive organs. — The sexes in most spe(;ie8 of the nemei'- 

 teans are separate, although a few foi-ms, Tetrastemma caecum, for 

 example, are hermaphroditic and some are ])rotandric. The genital 

 glands are simple sacs which develop in the body parenchyma, and 

 when the sexual products are mature open directly to the sui-face 

 of the body. In many forms which have broad, lateral, intestinal 



