140 INVEBTEBBATE MORPHOLOGY. 



Reproduction of the Turbdlaria. — Non-sexual reproduction 

 is not characteristic of the Turbellaria, though it occurs in cer- 

 tain Ehabdocoels. In Microstoma a transverse partition, con- 

 sisting of two closely- applied lamellse, forms, extending 

 from the outer wall of the body to the wall of the digestive 

 tract, which it constricts slightly without dividing. Later a 

 constriction of the outer surface of the body appears, the 

 two lamellae of the partition sejDarate slightly, and the indi- 

 vidual lying behind the partition develops a new mouth and 

 pharynx and a new brain, so that it resembles exactly the 

 anterior individual with which it is directly connected by the 

 uninterrupted digestive tract. Before these processes are 

 complete, however, they are repeated in each of the two indi- 

 viduals, so that a chain of four imperfectly separated individ- 

 uals results, and by further repetitions of the process chains 

 of 8, 16, or 32 individuals may arise, each provided with 

 mouth, pharynx, and brain, the anterior individual possessing 

 the original structures, and all connected by the digestive 

 canal which runs uninterruptedly through the entire chain 

 (see Fig. 28). Eventually the various individuals separate 

 from one another and become sexually mature. 



The sexual method, however, plays a much more important 

 part in the life-histories of the Turbellaria. The development 

 of the three lower groups has not as yet been as thoroughly 

 investigated as is desirable, but the phenomena which occur 

 in the Triclads, and especially in the Polyclads, have been fol- 

 lowed. The Triclads deposit their ova in chitinous cocoons, 

 which contain, besides the ova proper, large numbers of 

 amoeboid cells, originating in the vitellarium-pouches of the 

 parent, and serving as food for the young embryo. In asso- 

 ciation with this condition of affairs many peculiarities of 

 segmentation and growth occur in the Triclad embryos, all of 

 which must be considered as secondary adaptations. 



In the Polyclads, however, a more primitive state of affairs 

 occurs, the food-yolk being incorporated with the protoplasm 

 of the ovum, a more or less distinct irregular segmentation 

 resulting from its telolecithal arrangement (p. 53). The 

 diploblastic condition arises by an invagination either of the 

 embolic or epibolic type, but at an early period of the segmen 



