iv VERMES NERVOUS SYSTEM 215 



peculiar bodies whose significance is not yet clear are found floating 

 in it. In Phoronis also the body cavity is well developed, and lined 

 throughout by a peritoneal endothelium. At the most anterior end 

 of the body there is a septum which separates the cavity of the 

 prostomium and the tentacles from the body cavity. The descending 

 limb of the intestine is fastened to the body wall by a ventral 

 mesentery, which, on the limb which ascends forwards to the anus, 

 becomes a dorsal mesentery. The descending limb is further connected 

 with the body wall by 2 lateral mesenteries. Among the Bryozoa the 

 body cavity in the Pterobranchia and Endoprocta is extremely 

 reduced, but in the Ectoproda well developed. It is continued into 

 the tentacles, and is often lined with a ciliated endothelium, at least 

 this can be demonstrated in the fresh- water Bryozoa. The intestine is 

 fastened to the body wall on all sides by fibres which are considered to 

 be muscular. The gastric caecum is also suspended from the posterior 

 body wall by a strong non-muscular strand, the funieulus. In 

 Pahidicella there is also a second funieulus. In the Brachiopoda the 

 cavity containing the viscera is lined by an endothelium generally 

 ciliated over a great part of its surface. The enteric canal is fastened 

 to the body wall by a more or less complete dorso-ventral mesentery, 

 which, when complete (e.g. in Crania), divides the body cavity into two 

 lateral halves. There are often lateral membranes or bands as well 

 fastening the intestine to the body Avail a gastro-parietal band in the 

 region of the stomach, and an ileo-parietal band in the region of the 

 hind-gut. These bands have been compared with septa, which implies 

 that the Brachiopod body was originally composed of three segments. 

 This view is supported by other anatomical and ontogenetic facts. 

 The body cavity in the Brachiopoda is continued in the hollow spaces 

 of the mantle. An enclothelial lining of the body cavities of Piotatoria 

 and Dinophilus has not yet been proved. Distinct mesenteries and septa 

 are wanting. Fine fibres of connective tissue here and there connect 

 the organs lying in the body cavity with its walls ; no constant arrange- 

 ment, however, is found. 



The phylogenetic origin of the body cavity of the worms, and generally of the 

 higher Mctazoa, is not at present certainly established ; it is also impossible to say 

 decidedly how far the hollow spaces in the body, called body cavities, are homologous 

 in the various divisions of the Vermes. In the AiinnJutu and many other higher 

 animals it has long been known that special parts of the peritoneal epithelium are 

 the places of formation of the sexual products. AVe are therefore justified in asking 

 the question, whether the ovaries and testes of the lower acoelomous worms out of 

 whose germinal epithelium the sexual products are formed, do not correspond with 

 the chambers of the body cavity (ccelome) of the higher worms. 



VII. The Nervous System. 



The Nemertina (Figs. 140, 141). The central nervous system 

 consists of the brain, which is placed in front of or over the oesophagus 

 and under the anterior portion of the proboscidal apparatus, and of 



