THE TURBELLARIA 33 



nearly than any other the ancestor of the Turbellaria, and traces 

 the phylogeny of the Polyclads to the Ctenophora. 1 



In Lang's opinion all the constituent cells of the epidermis are 

 embedded in a nucleated interstitial tissue deprived of cell outlines. 

 ,The rod cells retain their primitive position in the epidermis 

 (Fig. IV. 2), although in Anonymns rhabdites and sagittocysts are 

 grouped to form " batteries " on the dorsal surface, which project 

 downwards into the parenchyma (Fig. IV. 3) ; in this form and 

 in Stylochoplana tarda true nematocysts with coiled threads occur. 



The chitinous spines of Enantia and Acanthozoon (8) are 

 specially worthy of mention. In the former they are marginal ; 

 in the latter on the dorsal surface. They suggest the chaetae 

 of Oligochaeta on the one hand, and the cuticle of Trematodes 

 on the other. Von Graff shows that each spine of Enantia (Fig. 

 XV. 5) commences as a cuticular secretion from a number of 

 epidermal cells raised up as a papilla ; around this spine, which 

 is hollow and golden -coloured, there are developed brownish 

 columns of chitin (?), each column being the product of a 

 single cell ; the whole group forms a broad base to the spine,, 

 causing it to resemble the prickle of a rose tree. 



In accordance with the greater size of the Polycladida, the 

 dermal musculature becomes more complex, there being as many 

 as six layers of alternating circular, diagonal, and longitudinal 

 muscles, variously arranged ; these, as in other forms, are more 

 strongly developed on the ventral surface. 



The parenchyma in Polycladida appears to differ from that v of 

 Rhabdocoela, in that the lacunae are intracellular, according to 

 Lang, who has traced their development. 



The mouth and pharynx may either retain the anterior 

 position of the ancestral Platyhelminth, or by differential growth 

 be thrust backwards to a central position, which is the more usual 

 condition in the Polyclad whilst in others it comes to lie near 

 the hinder end of the body. The pharyngeal sac is spacious and 

 the pharynx is almost universally a more or less folded, horizontal, 

 circular sheet of muscle, which arises from the circumference 

 of the sac (Fig. VI. 9, 12); only in the Euryleptidae and 

 Prosthiostomidae does it take on the form of a tubular pharynx 

 like that in Tricladida (Fig. XV. 8). The primitively simple 

 enteron which makes its appearance ontogenetically, becomes 

 at an early stage notched by the ingrowth of connective tissue 

 and dorso-ventral muscles, so that a greater or less number of 

 "caeca" are formed, and the varying extent to which this nipping 

 takes place leads to the persistence of a larger or smaller central 



1 See Willey, Q. J. Mic. Sci. xl., who places Ileteroplana n.g. in a new order 

 Archiplanoidea, formed for the reception of Ctenoplana and Coeloplana. See Part II. 

 of this work, "The Ctenophora," p. 16. 



3 



