168 THE NEMERTINI 



mesoblast formed from its wall splits, giving rise to the rhynchocoel ; the 

 inner layer becoming the musculature of the proboscis ; the outer layer 

 the wall of the rhynchocoel or proboscis sheath. 



The nervous system originates in two independent thickenings of the 

 epiblast on each side ; the dorsal lobe of the brain arises from the hinder 

 part of the cephalic disc ; the ventral lobe and the lateral nerve from the 

 somatic disc. The cerebral organ arises as a secondary pitting at the 

 anterior end of this latter disc (C). 



The excretory system is epiblastic in origin, arising as a pit on the 

 ventral surface of the larva, on either side of the stomodaeum (Figs. V., VI.). 

 This undoubted epiblastic origin of the excretory system is of considerable 

 general interest, since it confirms the view of Lang and others that the 

 system in Platyhelmia is epiblastic, and provides some confirmation of 

 Goodrich's distinction between a "nephridium" and a " coelomoduct" 

 The blood-vessels arise as a liquefaction of the mesoblastic jelly. 



The Classification of the Nemertines. Although the presence or 

 absence of an armature on the proboscis is no longer regarded as 

 of taxonomic value, yet the terms Anopla and Enopla may be used 

 as descriptive, and contrasting epithets, in the same kind of way 

 as " Invertebrata " and " Vertebrata," are 

 used. Since Biirger's classification is 

 founded on the relation of the nervous 

 system to certain tissues in the body wall, 

 it is necessary to describe a transverse 

 section of a Nemertine in this place, 

 and Carinella is chosen as an example. 



The epidermis consists of a single 

 layer of cells, of four kinds ciliated cells, 

 goblet cells, club-shaped gland cells, and 

 grouped gland cells (Fig. VII.) ; below it 

 is a basement tissue or cutis, then an 

 outer layer of circularly disposed muscle 

 fibres, followed by a deeper layer of longi- 

 .. D r,SSr fcudinal muscles (Fig. VIII.). The paren- 

 Burgerj. b, basement tissue ; d, chyma is thin in this genus, and below it 



ducts of gland cells (); dm, ,-11 * i i/ 



diagonal muscles (present only in 13 another layer of Circular muscles (con- 

 large species) ; e, ciliated epider- oirlprprl Vkir TTnKronVif ae arkmofin Vvnf Ivtr 



ma! ceils, produced internally into 8ia ered by ttubrecnt as somatic, but by 

 filaments; m, circular coat of Burger as splanchnic), closely investing 



muscles; n, nerve layer; p, , * * i 



grouped gland ceils ; r, radiating the enteroii and the rhynchocoel. In 

 the middle line, dorsally and ventrally, 

 some muscle fibres pass from this layer 

 to the cutis. The wall of the rhyncho- 

 coel consists of deeper circular muscles, to which longitudinal 

 muscles are added. 



In this transverse section the lateral and dorsal nerves (e t j) will 

 be seen lying outside the circular somatic muscles, immediately 



