RIBBON-WORMS 



233 



the gut and the body wall being filled up with gelatinous connective 

 tissue. In the larva?, however, a body cavity may be seen, either as an 

 archiccfile, i.e. the persistent segmentation cavity {IJneus obscurus), or 

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

 into two layers (Pilidium-larvse). In the adult only the blood spaces 

 and the cavity of the proboscis sheath are coelomic. 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. 126, n.). 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 communi- 

 cating internally either with 

 the brain itself or with ad- 

 jacent nervous tissue. In 

 those cases in which the de- 

 velopment has been studied, 

 these so-called lateral organs 

 arise from ectodermic insink- 

 ings and oesophageal out- 

 growths. In the most primi- 

 tive genus, Carinella, they 

 are absent, except in one 

 species. It has been sug- 

 gested that they conduce to 

 the respiration of the brain, 

 which is rich in haemoglobin, 

 and they have even been 

 compared with gill-slits. In 

 some forms the groove 

 through which they open to 

 the exterior is rhythmically 

 gested that they are sensory 

 are very sensitive; 

 ficial nerve plexus. 



eyes and eye spots are general ; and in some there are 

 otocyst-sacs. Apart from the cephalic $hts, 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) excretory region. 

 The mouth is ventral, and leads into a plaited glandular fore-gut or 

 cBsophagus, which is followed by a straight, ciliated mid-gut (stomach 

 and intestine), usually with regularly arranged lateral casca. Between 

 the caeca run transverse muscle partitions. The anus is in most cases 

 terminal. In a cavity along the dorsal median line there hes 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 



Fig. 128. — Transverse section of a simple 

 Nemertine {Carinella). — After Biirger. 



d.n., Dorsal nerve ; p.c, proboscis cavity ; g., 

 gut ; cm., circular muscles ; l.m., longitudinal 

 muscles ; d.v.m., dorso-ventral or diagonal 

 muscles ; l.v., lateral blood vessel. 



contractile. It has also been sug- 

 Apart from these organs, Nemertines 

 and in many this is associated with a super- 

 Tactile papillae and patches are often present ; 



