THE PLATYHELMIA 



Coelentera, for the greater number of nerve cells became aggre- 

 gated near that end of the body which is directed forwards during 

 movement to form a bilobed cerebral ganglion, lying near the dorsal 

 surface, anterior to the mouth, i.e. in the prostomium. From it a 

 network of nerve fibres spread in all directions ; but certain of 

 these strands of fibres became more important than others ; a pair 

 of ventral, a pair of lateral, a pair of dorsal, as well as anteriorly 

 directed nerves were thus distinguishable (Fig. II. 2, 4). But 





Fio. II. The Anatomy of an Ideal Platyhelminth. 



1. The alimentary and excretory systems, a, mouth ; 6, pharynx ; c, intestine ; d, main 

 excretory canals ; e, excretory pore ; /, flame cells. 



2. The nervous system, ventral view, b, cerebral ganglion ; c, ventral nerve tract ; d, 

 marginal or lateral nerve tract; e, dorso-lateral tract; /, medio-dorsal tract; g, male genital 

 pore ; h, female pore. 



3. The reproductive system, b, testis ; c, sperm duct ; d, penis ; e, prostate glands opening 

 into the lower part of the sperm duct ; /, antrum masculinum, in which the penis lies ; g, male 

 pore ; h, female pore ; i, ovary ; j, oviduct ; k, spermatheca or dilatation at the junction of the 

 two oviducts ; I, shell gland opening into this dilatation ; m, antrum femininum. 



4. A transverse section through the body, a, epidermis, below which is seen the layer 

 of circular muscles, represented by the continuous line ; below this, the layer of longitudinal 

 muscles, by a series of dots ; b, vertical, dorso-ventral muscles ; c, ventral nerve tract ; <f, 

 marginal tract ; e, latero-dprsal nerve tract ; /, medio-dorsal tract ; g, intestine ; h, parenchyma 

 (mesenchyme) ; i, testis ; j, ovary ; k, main excretory canal or duct. 



the nerve cells still retain a scattered arrangement, remaining at 

 various points of the general plexus, and do not give rise to ganglia 

 other than the brain. Sense organs were probably represented by 

 patches of pigment in the near neighbourhood of the brain, and 

 of widely distributed sensory cells. 



