v PHYLUM PLATYHELMINTHES 235 



wards. Each of these three main branches gives off numerous 

 smaller branches, which in turn become branched, so that the 

 whole intestine forms a ramifying system, extending throughout 

 the greater part of the body ; all the branches terminate blindly, 

 an anal aperture being absent. 



A system of vessels the water-vessels or vessels of the excre- 

 tory system (ex.] sends ramifications through all parts of the body. 

 There are two main, considerably coiled, pairs of longitudinal 

 trunks, right and left, which open externally on the dorsal surface 

 by means of several pairs of minute pores ; in front they are 

 connected together by a transverse vessel. The vessels of each pair 

 often join and separate again. Each main trunk gives origin to a 

 number of branches, which in turn give off a system of extremely 

 fine capillary vessels, many of which terminate in flame-cells (Fig 

 217, p. 264). A flame-cell is a nucleated cell having in its proto- 

 plasm a small space into which one of the capillaries leads ; in this 

 space lies a bundle of vibratile cilia, or a single thick cilium, which 

 performs regular undulating movements, giving it somewhat the 

 appearance of a flickering candle-flame. Cilia are said also to 

 occur in the course of some of the capillaries, though this is 

 doubtful. This system of vessels is usually regarded as excretory ; 

 but it may also have a respiratory function. 



A well-developed nervous system (Fig. 186) is present. At 

 the anterior end is a central knot of nerve-matter, the brain (br), 

 from which proceed backwards two longitudinal nerve-cords (I. ne.). 

 The brain consists partly of transverse fibres connecting together 

 the two longitudinal nerve-cords, partly of groups of nerve- 

 cells situated at the ends, or in the course of, the nerve-fibres. 

 The nerve-cords give off both internally and externally numerous 

 transverse branches, which divide into finer twigs ; the internal 

 branches of the two cords frequently anastomose, thus forming 

 commissures or connecting nerve-strands between the two. A 

 number of nerves extend forwards to the anterior margin, which 

 is highly sensitive. 



Reproductive System. The reproductive organs (Fig. 187) 

 are hermaphrodite or monoecious in their arrangement, both male 

 and female organs occurring in the same individual. The genital 

 aperture leads into a small chamber, the genital atrium or cloaca, 

 which is common to both the male and the female reproductive 

 systems. 



The male part of the apparatus consists of testes, vasa deferentia, 

 and penis. The testes (tes.) are numerous rounded bodies, 

 situated near the right and left borders. Two ducts, the right 

 and left vasa deferentia (v.d.), run backwards from the neighbourhood 

 of the testes and unite in the middle line posteriorly. The median 

 duct formed by the union of the two vasa deferentia traverses a 

 protrusible muscular organ, the penis (p), to open into the genital 



