i6o WORMS. 



The nervous system consists of two cerebral ganglia and 

 two lateral nerve-cords. In most forms the cords he along the 

 sides, but sometimes they approach one another ventrally, 

 and in one or two cases dorsally. There is sometimes a 

 supra-anal union of the lateral cords, and there is an anterior 

 commissure around the proboscis. Simple eyes and other 

 sensory structures are present. Very characteristic of 

 Nemerteans are two ciliated slits on the sides of the head. 

 They sometimes extend as far inwards as the brain, and have 

 possibly a respiratory function, as their slight resemblance to 

 gill-slits and the occurrence of haemoglobin in the nerve- 

 substance of many Nemerteans (Schizonemertea) suggest. 



The Nemerteans seem to be the simplest animals with 

 a distinct blood-vascular system. Its cavities represent the 

 remnants of a ccelome. In some, the blood has distinct 

 corpuscles ; in Drepanophorus, these contain haemoglobin. 

 One would think that the evolution of a respiratory pigment 

 so efificient as hemoglobin would soon induce other steps 

 of progress. 



Excretory organs are represented by two anterior kidney- 

 tubes or nephridia. The reproductive organs are very simple ; 

 the animals are almost all unisexual. 



The development is often indirect, thus Lineus has a 

 creeping ciliated larva (the larva of Desor), and a commoner 

 free-swimming helmet-like type is known as a Pilidium. 



Classification. 



Order I. Palaeonemertea. The head is without deep lateral grooves. 

 The proboscis is without spines. The mouth lies behind the 

 brain. 



e.g., Carinella ; Folia. 



Order 2. Schizonemertea. The head has a deep cleft on each side. 

 The proboscis is without spines. The mouth hes behind ihe 

 brain. 



e.g. , Linens, some species of which attain great length ; Cere- 

 bratulus ; Langia, with nerve-cords which approach 

 dorsally. 



Order 3. Haplonemertea. The head is without deep lateral grooves. 

 The proboscis is armed with a spine, or with several spines. 

 The mouth is usually in front of the brain. 



Amphiporus; Drepanophorus, with oval blood-corpuscles 

 containing haemoglobin, and nerve-cords which 

 approach ventrally ; Nemertes. 



