280 PLATHELMINTHES. 



mm. broad and 0.01 mm. long; the ripe proglottids at the end are 

 elongate oval, 5 mm. broad and 12 mm. (half an inch) long. 



Head and proglottids have certain common characters. Their 

 connective-tissue parenchyma contains numerous spherical con- 

 cretions of lime, and consists of cortical and medullary sub- 

 stance. The first contains to a marked degree the muscles, the 

 latter the other organs. Nerves and water-vascular system extend 

 through the whole length of the worm. In the head is the paired 

 cerebral ganglion of the flatworms (fig. 239), sometimes fused to a 

 single mass by the great development of the commissure or partially 

 concealed by accessory parts connected with attachment (fig. 

 242). From the brain two principal nerves run backwards, usually 

 near the edges of the proglottids (fig. 244, N). The water-vascu- 

 lar (excretory) system begins with a capillary network richly 

 provided with flame cells. It extends through head and proglottids ; 

 usually four main trunks are present, two being less developed and 

 it is possible are sometimes absent. The two chief trunks are fre- 

 quently connected by a cross-trunk on the hinder margin of each 

 proglottid (fig. 244). The system opens on the posterior edge of 

 the last proglottid, but accessory mouths may occur on other 

 proglottids. 



The scolex and proglottids are distinguished by the facts that 

 the proglottids contain the sexual organs, while the scolex bears 

 the anchoring apparatus, for the latter has, besides producing 

 proglottids, to fasten the worm in the intestines. Most important 

 of the adhesive organs are the suckers (acetabula) ; less important 

 are the hooks, which, in numbers, are either arranged in a circle or 

 are borne on protrusible and retractile probosces (fig. 240242). 



When a circle of hooks is present it is on the anterior end and is moved 

 by a special apparatus, the rostellum. This is a plug of complexly arranged 

 muscles (fig. 242) which can arch and flatten the central area. In many 

 species the arching is increased by a muscular sheath, the flattening by 

 retractors. Each hook has its point outwards and its base with two roots, 

 one of which rests on the rostellum; the protrusion of the rostellum forces 

 the points outwards into the mucous membrane of the intestine. In some 

 Tcenice without the circle of hooks (T. saginata) the rostellum is replaced 

 by a sucker-like depression. Since the rostellum arises in development 

 from a similar cup, it may be a modified apical sucker, but it is doubtful 

 how far comparisons may be made with the oral sucker or the alimentary 

 tract of the trematodes. 



The sexual organs are hermaphroditic and are present in num- 

 bers equal to those of the proglottids, so that these were formerly 



