204 



PLATHELMINTHES 



of which can be everted and thrust forward in the form of a proboscis 

 (Fig. 333) : in most hoplonemerteans this opening coincides with the 

 moutli. The proboscis is often nearly as long as the body itself and can 

 be thrust out far in advance. It is primarily a tactile organ and in the 

 hoplonemerteans is armed with calcareous stylets of characteristic form, 

 indicating that it has also an offensive function. Dorsal to the proboscis 

 in most species are the openings of the so-called cephalic glands. In 

 Malacohdella a large sucker is present at the hinder end, and Nectonemertes 

 possesses a pair of lateral swimming 

 organs. Many heteronemerteans have 

 a caudal cirrus at the posterior end. 

 The outer surface of the body is a 

 glandular, ciliated epithelium and is 

 often brightly colored. 



No body cavity is present, the 

 spaces between the organs being filled 

 with a gelatinous parenchyma. The 

 pi'oboscis, however, is surrounded by a 

 muscular sheath containing a corpuscu- 

 lated fluid (Fig. 333,4). The digestive 

 canal extends the whole length of the 

 body, and is usually differentiated into 

 (Esophagus, stomach, intestine, and rec- 

 tum. The intestine is often provided 

 with regularly recurring paired diver- 

 ticula and in the hoplonemerteans has 

 a long intestinal caecum, extending for- 

 ward to near the front end of the body. 

 Two or three longitudinal blood vessels 

 with connecting branches and contain- 

 ing a corpusculated fluid differing 

 somewhat from that in the proboscis 

 sheath extend the length of the body and are connected with large blood 

 spaces. A pair of profusely branched longitudinal excretoiy canals lie 

 alongside the lateral blood vessels in the anterior portion of the animal, 

 which usually open to the outside through one or more pores on each side 

 of the body. Minute branches of these canals extend into the parenchyma 

 and end in flame cells. The central nervous system consists of a four- 

 lobed brain and a pair of large lateral nerves (Fig. 332) extending to 

 the hinder end of the body, where they join; a dorsal median nerve is 

 also usually present and in some species a ventral median one as well. 

 A pair of lateral ciliated canals called the cerebral organs because they 



Fig. 333 — Diagram of nemertean 

 worm — dorsal aspect. A, with pro- 

 boscis retracted ; B, with proboscis 

 extended (altered from Boas). 1, 

 stylet; 2, proboscis; 3, poison 

 gland ; 4, proboscis sheath ; 5, in- 

 testine ; 6, gonads. 



