8 4 



THE TREMATODA 



whose development has been followed, the intestine is at first sac- 

 like. The two main limbs may also carry lateral caeca, which 

 appear to have developed independently of those in Triclads and 

 Polyclads, and which ramify amongst the lobes of the laterally 

 placed vitellaria. It is necessary to point out that the intestine of 

 Distomum hepaticum, with its multitude of branching caeca, is quite 

 exceptional amongst the family Distomidae, and, indeed, amongst 

 the Malacocotylea ; for in this order the gut is characteristically 

 bifurcated, though the length of limbs may vary in different 

 species. 



The excretory system presents little of the network character 



seen in many Turbellaria; the ar- 

 rangement of the canals and the 

 position of the pores is seen in 

 the diagrams. Apparently, the 

 primitive position of these pores 

 is not posterior ; separate pores 

 exist in cercariae and rediae ; 

 the median pore of the adult 

 Malacocotylean and Aspidogaster 

 being due to a fusion of the 

 right and left ducts. 



With regard to the nervous 

 system, which in its main lines 

 was first correctly recognised by 

 Ramdohr (1814), a comparison 

 of the figures will show that in 

 the Heterocotylea a more primi- 

 tive arrangement persists than 

 in the Malacocotylea, in which 

 there is a remarkable constancy 

 of longitudinal stems and cir- 

 cular commissures (Figs. XX, 

 and XXL). 



Some of the ectoparasitic 

 forms, as well as the free larvae 

 of the endoparasitic forms, 

 possess " eyes " which, however, 

 no. xx. compared with those of Turbel- 



Nervoussystemofz>.c2/Kttdraceim >a satype laria are in a degenerate condi- 



of that of the Malacocotylea (from Looss). The fion Thp ran of rnVrnpnt i<* 

 animal is viewed from above, o, brain ; b, uon< [ P 1 ^ 111 ' 



dorsal longitudinal nerve stem ; c, lateral directed externally, and embraces 



nerve ; d, ventral nerve. These three stems 1-1 * ,11 



are connected by the circular commissural a Spherical rciringent body Or 



ouS. 1 " 5 ' e ' anterior ^' ventralsuckersin "lens," which in its turn abuts- 



upon a ganglion cell, a branch 

 of which runs to the brain. There is nothing which can be 



