TYPE PLATYHELMINTHES. 169 



the Nemerteans perhaps show greater similarities to Alloiocoelan Turbel- 

 laria than to Polyclads. Perhaps an explanation of the process is to be 

 found in the sloughing of the ectoderm and the formation of new ciliated 

 cells which is seen in the larva of a Palseonemertean, Cephalothrix, the 

 metamorphosis of Desor's larva and of the Pilidium being a greater and 

 more complicated ecdysis derived from the simpler one. 



Some interesting evidence as to the morphological significance of the 

 anus is to be derived from a study of its development in the Nemerteans. 

 It is an opening which has been considered by some to have arisen by the 

 closure in the middle of an elongated slit-like blastopore the two ends, how- 

 ever, remaining open to form respectively the mouth and anus; and it has 

 been thought that the direct transformation of the blastopore into the per- 

 manent mouth in some cases, and in others into the permanent anus, 

 receives on this theory an explanation. The phenomenon of the closure of 

 the blastopore in the middle does actually occur in the Annelid-like Tra- 

 cheate Peripatus, and in many forms both mouth and anus stand in close 

 ontogenetic relationship to the blastopore. In the Nemerteans are repre- 

 sented the most lowly organized animals which possess both mouth and 

 anus, and accordingly it might be expected that in them the original rela- 

 tionships will be most clearly seen. The young Nemertean possesses no 

 anus. It resembles, so far as its digestive tract is concerned, an Alloiocoelan; 

 it is only relatively very late in its life-history that the anus appears, and 

 then in a region of the body which has no relation whatever to the original 

 blastopore. This fact should carry considerable weight with it, especially 

 as in the majority of forms the anus is, in comparison with the mouth, 

 of relatively late formation. It seems not improbable that primitively it 

 had no relation with the blastopore, and where such relations do occur 

 they are entirely secondary. 



The indication of metamerism seen in the Nemerteans needs no further 

 discussion after what has been said on p. 43 with reference to similar pecul- 

 iarities in the Turbellarians. 



SUBKINGDOM METAZOA. 



TYPE PLATYHELMINTHES. 



I. Class TURBELLARIA. Ectoderm ciliated; no anal opening. 



1. Order Acoda. Mouth present, but no digestive tract. Convoluta. 



2. Order Alloioccela. Digestive tract present; space between it and 



body-wall occupied by parenchyma. Monotus, Playiostoma. 



3. Order Rhabdoccela. Digestive tract straight rod- or sac-like; 



space between it and body-wall not filled with parenchyma. 

 Microstoma, Mesostoma, Prorhynchus, Vortex. 



4. Order Tridadea. Digestive tract branched, three principal limbs 



giving rise to secondary branches; male and female reproductive 



