THE TURBELLARIA 15 



such forms as Convoluta and Haplodiscus there is occupying the central 

 region a compact mass of nucleated protoplasm, in which cell out- 

 lines are not distinguishable (Fig. V. 7, 8). In this axial syncytium 

 the remnants of prey are found ; it is the " digesting parenchyma," 

 or more properly, the syncytial hypoblast ; in Proporus and others 

 this digesting parenchyma is represented by separate amoeboid 

 cells, which extend throughout the central parenchyma in this 

 latter case, the hypoblast cells which in the embryo surround a 

 true enteron, have wandered in all directions ; in the former case, 

 the cells have fused to form a more concentrated syncytium. 



In Convoluta Schultzii, Cortex viridis, and a few other species, 

 chlorophyll bodies or (in G. paradoxa) yellow cells occur in the peri- 

 pheral parenchyma ; it has been shown experimentally that these 

 bodies behave like the chlorophyll bodies of green plants, and they 

 appear to be of considerable importance to the animal, which then 

 presents the " holophytic " mode of nutrition. How far these 

 bodies, which are similar to those occurring in Anthozoa, to which 

 Brandt has given the name Zoochlorella, are part and parcel of the 

 animal, or whether they are symbiotic algae is still a matter of 

 dispute. Haberlandt (28) finds that they are nucleated, but with- 

 out a cell wall (Fig. V. 3, 4, 5) ; that when isolated they cannot 

 form a cell wall, and soon die, and in fact are unable to lead an 

 independent existence. They appear to be algae, or flagellata 

 (similar to Chlamydomonas), but so adapted are they to a symbiotic 

 existence, that they now form a definite and inseparable part of 

 the tissues of the worm and function as assimilating organs, at 

 the same time providing, by their disintegration, food for the 

 Planarian (Geddes, 19). 



The nervous system in the primitive Turbellarian was no 

 doubt similar to that which occurs in the Polycladida, viz. a net- 

 work of nerve fibres and cells which had already sunk below the 

 dermal muscles, arising from or converging to a definite group of 

 ganglion cells forming a "brain" near the anterior end of the 

 body. The nervous system as presented by the majority of 

 Rhabdocoela has lost its ancestral character of a network ; there is 

 a pair of well-developed cerebral ganglia near the anterior end of 

 the body, whence four pairs of nerves arise, of which one pair 

 lying along the ventral surface is especially stout : other nerves go 

 forwards. In tmTAlloiocoela a few transverse commissures may 

 occur between these nerves ; whilst in Acoela, in which a network 

 occurs (Fig. V. 1), the nervous system, apart from the brain, 

 presents a comparatively primitive condition. There may also 

 be a nerve plexus in the wall of the pharynx. The brain was 

 first correctly interpreted as such by Ehrenberg, 1836. It had 

 previously been regarded as the "heart," while Duges held the 

 nerves for " blood-vessels." 



