GENERAL ACCOUNT OF NEMERTEA. 



199 



d.n 



as a schizocoele, i.e. a. space formed by the cleavage of the mesoderm 

 into two layers (//&*if/;/-larvae). In the adult only the blood spaces 

 and the cavity of the proboscis sheath are ccelomic. The nervous 

 system consists of a brain generally four-lobed, the two lobes of eacH" 

 side being closely united and connected with those on the other side by 

 a commissure above and by another below the proboscis cavity. From 

 the lower lobes two longitudinal nerve-stems run along the sides, and 

 are sometimes united posteriorly above the anus (Fig. 104, /..) In 

 some forms there is in addition a dorso-median nerve, and sometimes a 

 ventro-median nerve. 



On each side of the head there is a ciliated pit communicating with 

 the exterior through an open slit or groove, and communicating inter- 

 nally either with the brain itself or with adjacent nervous tissue. In 

 those cases in which the development has been studied, these so-called 

 lateral organs arise from 

 epiblastic insinkings and 

 oesophageal outgrowths. In 

 the most primitive genus, 

 Carinella, they are absent, 

 except in one species. It 

 has been suggested that they 

 conduce to the respiration of 

 the brain, which is rich in 

 hcemoglobin, and they have 

 even been compared with 

 gill-slits. In some forms the 

 groove through which they 

 open to the exterior is rhyth- 

 mically contractile. It has F IG. 105. Transverse section of a simple 

 also been suggested that they Nemer ? ean ( Cari**//*). After Burger, 

 are sensory. Apart from 



these organs, Nemertines are d - n -> Dorsal nerve; AC., proboscis cavity ; g., 

 very sensitive and in many ^^^^Tfe^S^ 

 this IS associated With a super- diagonal muscles ; l.v., lateral blood vessel, 

 ficial nerve plexus. Tactile 



papilloe and patches are often present ; eyes and eye spots are 

 general ; and in some there are otocyst-sacs. Apart from the cephalic 

 slits, the head also bears sensory pits and grooves and terminal 

 sensory spots. In some there is a pair of lateral sense organs 

 in the (anterior) nephridial region. The mouth is ventral, and leads 

 into a plaited glandular fore-gut or oesophagus, which is followed by a 

 straight, ciliated mid-gut (stomach and intestine), usually with regularly 

 arranged lateral cseca. Between the cseca run transverse muscle parti- 

 tions. The anus is in most cases terminal. In a cavity along the dorsal 

 median line there lies the remarkable proboscis. It is protruded and 

 retracted through an opening above, or, in a few cases, within the 

 mouth. It arises in the body-wall and is surrounded by a cavity 

 (rhynchocoelom) bounded by a muscular proboscis sheath. The pro- 

 boscis is a muscular, richly innervated tube lined with glandular epi- 

 thelium, sometimes protruded with such force that it separates from the 

 body. It has been compared in its retracted state to a glove-finger 



