TYPE PLATYHELMINTHES. 163 



the exterior, the amis, an opening unrepresented in other 

 Platyhelminths. The digestive tract is no longer a blind 

 sac, but has the form of a tube, as in all the higher types. 



In the anterior end of the body, above the digestive tract, 

 is a structure, the proboscis (Fig. 84, pr), essentially peculiar 

 to the Nemerteans, although indications of such an organ are 

 to be found in the Rliabdocoda. It consists of a closed tube, 

 the proboscis sheath, with muscular walls, imbedded in the 

 body parenchyma and extending backwards in some cases 

 almost to the end of the body, and within it lies the proboscis, 

 also a tube, united to the wall of the sheath near its anterior 

 end and in fact closing it at that region. From this line of 

 attachment the proboscis stretches back in the cavity of the 

 sheath, the space between it and the walls of the sheath being 

 filled with fluid. It is a simple imagination into the cavity 

 of the sheath of the external body-wall, whose musculature 

 as well as ectoderm are continuous with that of the proboscis. 

 From the tip of the invaginatiou a band of muscle fibres, 

 forming the retractor muscle (rm) of the proboscis, passes to 

 the wall ol the body. By the contraction of the muscular 

 walls of the sheath the fluid contained in its cavity forces the 

 proboscis to be evaginated sometimes with sufficient force 

 to tear itself loose from its line of attachment ; but should 

 this accident not happen, the proboscis can be reinvaginated 

 by the contraction of its retractor muscle. The function of 

 this organ is doubtful. In some cases it is undoubtedly a 

 weapon of offence and defence ; but it seems not improbable, 

 from its rich nerve-supply and from the probable function of 

 its prototype in the Khabdoccela, that in some cases at least 

 it may be a tactile organ. 



A well-developed nervous system is always present, though 

 it may show in some cases, as Carinella, the primitive character 

 of being still imbedded in the ectoderm or else lying immedi 

 ately beneath it. In other cases, however, as Cerebratulus, it 

 is enclosed in the muscles of the body-wall or may even be 

 completely within them, imbedded in the parenchyma. It 

 consists in its most usual form of two ganglionic masses 

 (Fig. 84, ce) from which short nerves pass forwards and which 

 are united by two transverse commissures, one of which passes 



