23S 



PLATHELMIXTHES 



a periL,'astric sinus, arising by a separation of the intestinal layers, forms a part 

 of the circulatory system. 



The neryous system of the 'worms' has in common a pair of supra-ccsopha- 

 geal ganglia ('brain') \yhich sends out two strong longitudinal cords (to which 

 others may be added) which must be regarded as part of the central nenous 

 system since they bear ganglion cells. These cords may be lateral or on either 

 side of the mid-Ventral line. In the latter case those of the two sides may be 

 united at regular interyals, thus giying the ladder type (p. 113), the yentral chain 

 being connected with the brain by cords on either side of the oeso])hagus. This 

 neryous system, always ectodermal in origin, may be epithelial, forming part 

 of the skin, or it may sink to different depths in the other tissues. 



Fig. 211. — Trochophore (Loven's larya) of Poly(;&rdius (from Hatschekl. .4. 

 anus: dLM, dorsal muscles; ED^ hind gut; J, stomach; J,, intestine; Mslr^ mesodermal 

 band; n, neryes; A'cplt, protonephridia; O, mouth; Oe, a?so|.ihagus; ocLM ^ oesophageal 

 muscle; SP, apical plate; vLM , yentral muscle; vLN, lateral nerye; Wkr,vkr, pre- and 

 post-oral zones of cilia; Tr5, apical cilia; li'G, adoral cilia. 



A peculiar type of larya appears in yarious groups of inyertebratcs, being 

 recognizable in modified forms in echinoderms and molluscs as well as in worms. 

 This is the trochophore (fig. 211) a gelatinous ball trayersed by fore, mid and hind 

 gut. At first the body is uniformly ciliated, but the cilia are later restricted 

 to definite tracts. One of these ciliated bands is pretty constant, jiassing around 

 the body in front of the mouth, thus marking off an apical region, in the centre 

 of which is an epithelial thickening, the anlage of the brain, often marked by a 

 bundle of cilia. Besides mesenchymatous muscle fibres, one or inore pairs of 

 protonephridia may be present. In the corresponding lar\a of many of the 

 flat worms (the prolrocliula, fig. 216) hind gut and protonephridia are lacking. 



PHYLUIM IV. PLATPIELIMINTHES (PLATODES, Flatworms). 



This group is well characterized by the names. AA'ith fc^y exceptions 

 (rhabdococles, many tremtitodes) the nearly flat yentral surface and the 

 slightly arched back pass \yith a more or less sharp margin into each 

 other (fig. 212). In many cases the yentral surface is distinguished by 

 its lighter color. In all the bilaterally symmetrical body, without ccvlom, 



