340 PLATTHELMINTHES. 



traded, it is inverted like the finger of a glove, so that the blind 

 end at which the spines are placed becomes the extreme front end 

 of the protruded proboscis. 



The brain attains a considerable development. Its two halves are 

 connected by a double commissure which embraces the proboscis, and 

 in them several lobes, usually a dorsal and ventral, may be distin- 

 guished. The two ventral lobes are produced into the two lateral 

 nerve trunks, which in certain cases (Oerstedtia} may approach 

 each other on the ventral surface. The nerve trunks contain not only 

 fibres but also a superficial layer of ganglion cells, which may give 

 rise to ganglion-like enlargements at the points of exit of the nerve 

 branches. In the embryos of Prosorochmus Olaparedii the nerve 

 trunks are said to end in an enlargement. In the cephalic region 

 there are two strongly ciliated depressions known as the cephalic 

 slits, beneath which special lateral organs, supplied with nerves from 

 the brain or it may be posterior lobes of the brain itself, are placed. 

 These structures are probably sense organs. The cephalic slits were 

 formerly erroneously taken for the openings of respiratory organs. 

 Eyes are widely distributed, and usually consist of simple pigment 

 spots which rarely contain refractive bodies. Exceptionally, as 

 in Oerstedtin pallida, two otolithic vesicles are found on the 

 brain. 



The Nemertines, unlike all other Platyhelminthes, possess a blood- 

 vascular system. This consists of two sinuous lateral vessels in 

 which the blood flows from before backwards, and a straight dorsal 

 vessel in which the blood flows in the reverse direction. This latter 

 is connected with the ventral vessel at the posterior end of the body 

 and in the region of the brain by wide loops, and in the rest of its 

 course by numerous narrower transverse anastomoses. These vessels 

 lie in the body cavity and have contractile walls. The blood is 

 usually colourless, but in some species it is red. In Amphiporus 

 sptendens, Borlasia splendida, the red colour (haemoglobin) is con- 

 tained in the oval disc-shaped blood corpuscles. 



The Nemertines are, with some few exceptions (Borlasia herma- 

 phroditica\ dioecious. The two kinds of. generative organs have the 

 same structure, and are sacs filled with ova or spermatozoa lying in 

 the lateral portions of the body between the pouches of the intestine, 

 and opening to the exterior by paired openings in the body wall. 

 The ova, when laid, frequently remain connected by a gelatinous 

 substance, and are deposited in irregular masses or in strings, from 

 the middle of which the animal creeps out, like the leech out of its 



