238 



PLATHELMINTHES 



a perigastric sinus, arising by a separation of the intestinal layers, forms a part 

 of the circulatory system. 



The nervous system of the 'worms' has in common a pair of supra-cesopha- 

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

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

 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 intervals, thus giving the ladder type (p. 113), the ventral chain 

 being connected with the brain by cords on either side of the oesophagus. This 

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

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



Mi 



FIG. 211. Trochophore (Loven's larva) of Polygordius (from Hatschek). A. 

 anus; dLM, dorsal muscles; ED, hind gut; J, stomach; J,, intestine; Mstr, mesodermal 

 band; n, nerves; Neph, protonephridia; O, mouth; Oe, oesophagus; oeLM, oesophageal 

 muscle; SP, apical plate; vLM, ventral muscle; i<LN, lateral nerve; Wkr,wkr, pre- and 

 post-oral zones of cilia; WS, apical cilia; wz, adoral cilia. 



A peculiar type of larva appears in various groups of invertebrates, being 

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

 This is the trochophore (fig. 211) a gelatinous ball traversed 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, passing 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 more pairs of 

 protonephridia may be present. In the corresponding larva of many of the 

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



PHYLUM IV. PLATHELMINTHES (PLATODES, Flatworms). 



This group is well characterized by the names. With few exceptions 

 (rhabdocceles, many trematodes) the nearly flat ventral surface and the 

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

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

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



