318 SYNOPTIC COLLECTION. 



the segmented condition is characteristic of larval stages 

 and consequently of ancestral forms, and is lost through 

 semiparasitic and parasitic habits. 



Fourthly : The Turbellaria have a complicated diges- 

 tive system. The reproductive organs are also complex, 

 as shown by Gamble. 1 The embryo is retained in the 

 uterus and the young resemble very early the adult. 



Fifthly : When the trochophore larva of an Annelid is 

 compared with the Pilidium larva of a Nemertean or 

 with Miiller's larva of a Turbellarian (see Pis. 727, 782, 

 789), the Annelid trochophore is the more primitive so 

 far as we are able to judge. This is, as we have seen, a 

 spherical body surrounded by bands of cilia and contain- 

 ing a digestive system with a mouth and an anus. This 

 trochophore passes into the worm by changes which are 

 comparatively simple and easily followed. The Pilidium, 

 on the other hand, has a bell-shaped part from which 

 hang two lobes. The mouth opens between the lobes 

 and the anus ends blindly. Within this organism the 

 worm is formed by complex changes, after which it leaves 

 the Pilidium and the latter continues to live some time.' 2 



Miiller's larva, as will be seen (PI. 789), is elongated in 

 shape and is provided with eight finger-like prolonga- 

 tions. Numerous changes are undergone before this 

 larva assumes the adult form (see p. 319). 



Sixthly : Most Chaetopods pass through a trochophore 

 stage while only a comparatively small number of Tur- 

 bellaria (the Cotylea and a few Acotylea of the Polyclads) 

 go through the stage represented by Miiller's larva, while 

 the rest of the Polyclads. the Triclads, and the Rhabdo- 

 coela have either a modified Miiller's larva or are without 

 this larval form, the stage being skipped altogether 

 through acceleration in development. The remaining 

 classes of the Platyhelminthes (Trematodes, Cestodes) 



1 Quart. Journ. Micr. Sci., XXXIV, 1893, p. 438. 



2 Balfour, Comp. Embryol., I, 1880, p. 167. 



