NEMERTEA 207 



proboscis to aid in burrowing. The part of the proboscis in front of 

 the brain is called the rhynchodaeum. 



The ectoderm is completely ciliated : there are gland cells amongst 

 the ciliated epithelium; within this are layers of, first, circular, and 

 then longitudinal, muscles. There is a complete nervous sheath, 

 which in the most primitive nemerteans lies at the base of the ecto- 

 derm cells, in others between the circular and longitudinal muscles, 

 and in the most advanced forms within both layers of muscle. While 

 the nervous system is thus extremely primitive there are concen- 

 trations to form lateral nerve cords and a pair of cerebral ganglia 

 above the mouth, each cerebral ganglion being divided into a dorsal 

 and ventral lobe and connected by commissures above and below the 

 proboscis sheath. The dorsal lobe is subdivided into an anterior and 

 posterior part: the posterior part is in close relation with an ecto- 

 dermal pit, the cerebral organ, which is situated in some forms in 

 a lateral slit. There are occasionally eyes of simple structure. 



Inside the muscle layers the body is filled with parenchyma like 

 that of the Platyhelminthes (Fig. 162 B), but in it are one, two or 

 three longitudinal vessels, connected together by transverse vessels 

 with contractile walls, which constitute the vascular system. The blood 

 is generally colourless, but has corpuscles which sometimes contain 

 haemoglobin. The circulation is assisted by the movements of the 

 body. It can hardly be supposed that the blood system, situated so 

 deeply in the body, can be respiratory in function. 



The alimentary canal is a straight tube, the mouth and anus being 

 nearly or quite terminal. The excretory system is formed by a pair 

 of tubes situated laterally, each of which communicates with the 

 exterior by one or several pores and gives off many branches, ending 

 internally in flame cells like those of the Platyhelminthes. In some 

 cases the end organs come into contact with the blood vessels. The 

 generative organs are series of paired sacs alternating with the 

 pouches of the mid gut and these each develop at the time of maturity 

 a short duct to the exterior. 



Most nemerteans develop directly, but in some a pelagic larva with 

 a remarkable form of metamorphosis is found. This larva is known as 

 the Pilidium (Fig. 163). A conical gastrula with a flattened base is 

 first formed by invagination and it passes into the Pilidium by the 

 following changes. A band of cilia round the base constitutes the 

 prototroch and forms the locomotory organ of the larva ; it is drawn 

 out into two lateral lappets. An apical sense organ is formed by a 

 thickening of the ectoderm. Two cells migrate into the blastocoele and 

 break up into a tissue called mesenchyme, which is partly converted 

 into larval musculature and partly remains undifferentiated until 

 needed as raw material for the adult organs. The gut is connected 



