VI] ANATOMY 115 



There is a well-defined central nervous system amongst Nemer- 

 tines, consisting of a pair of ganglia lying at the front of and above 

 the mouth less frequently at the sides of it and connected by a 

 nervous band or commissure which passes above the proboscis- 

 sheath. The hinder parts of the ganglia are more or less distinctly 

 separated as posterior lobes, and come into close relation with 

 curious pouches of invaginated ectoderm, termed cephalic pits 

 (Fig. 49), which seem in some cases at any rate to subserve the 

 respiration of the nervous tissue, for the latter in some species 

 contains haemoglobin. The ganglia are continued backwards into 

 two powerful nerve-cords lying at the sides of the body. In the 

 lower species these cords can be seen to lie just beneath the 

 ectoderm and to be but thicker portions of a sheath of nerve-fibrils 

 extending all round the body, and derived from the bases of the 

 ectoderm cells. In the higher species the nerve-cords lie well within 

 the muscles, and this sheath is not so evident. The principal 

 resemblance between Platyhelminthes and Nemertinea lies in the 

 arrangement of these ganglia and lateral nerve-cords. 



Sense-organs in the form of eyes of simple structure often occur 

 immediately over the region of the ganglia. 



The interstices between the various organs inside the muscles 

 are filled up with parenchyma like that of Platyhelminthes, but 

 there exist three longitudinal tubes with well-defined walls which 

 are regarded as blood-vessels. Two of these tubes lie at the sides 

 of the alimentary canal, and one is situated above it and below the 

 proboscis-sheath. These vessels are connected anteriorly by arches 

 and they unite with one another in both the head and tail. These 

 blood-vessels are to be regarded as last remnants of the cavity which 

 in the embryo intervenes between ectoderm and endoderm, which in 

 Coelenterata is filled with almost fluid jelly (containing in Aurelia 

 99 /o f water). This cavity is called by various names. By em- 

 bryologists it is termed the blastocoele or segmentation cavity, 

 because it appears early in development by the mutual separation 

 of the blastomeres or first cells resulting from the division of 

 the fertilised egg. By students of adult anatomy it is termed 

 haemocoele (Gr. cu)aa, blood) because the fluid which it contains is 

 the first form of blood, but the best and simplest name for it is one 

 coined in Germany, viz. primary body-cavity. 



The excretory organs have the form of a pair of branched 

 nephridia opening at the sides of the body not far behind the 

 cephalic pits. Their branches end blindly and the terminations 



82 



