THE CTENOPHORA 19 



satisfactoiy evidence of the relationship of Platyhelminthes to 

 Ctenophora, still less of the descent of the former group from the 

 latter. The most that can be said is that Ctenoplana and Coeloplana 

 afford an interesting suggestion as to how the Polyclada might 

 conceivably have been derived from a Ctenophore-like ancestor. 

 But whilst we decline to attach very much importance to the 

 resemblance between Ctenoplana and the Polyclada, we cannot 

 ignore other points of resemblance between the Ctenophora and 

 the Platyhelminthes. The earlier stages of segmentation, the 

 formation of the gastrula, the outgrowth of the primitive mesoderm 

 cells into four mesodermal bands placed crosswise, and the forma- 

 tion of the mesenchymatous mesoderm from these bands, are 

 features in which the young Polyclad resembles the young Cteno- 

 phore in a remarkable degree. The gelatinous mesoderm of 

 Ctenophora, with its layers of longitudinal, transverse, and 

 radiating branched muscle fibres, most nearly resembles the 

 mesenchyme of Turbellarian worms, and the ciliated larvae of 

 many Platyhelminthes, more particularly the Pilidium larva of 

 Nemertines and the larva of Stylochus pilidium, Avith its uniform 

 coat of cilia, its aboral sense organ, its stomodaeum or pharynx, 

 and its enteron lined with endoderm cells, are most suggestive of 

 the hypothetical ancestor from which both the Turbellaria and 

 the Ctenophora may have originated. The conclusion is that the 

 Turbellaria, the Nemertines, and the Ctenophora are descended 

 from a common ancestor which is most nearly represented by the 

 larva of Stijlochus. Such an ancestor would be spherical or hemi- 

 spherical in shape, would have an aboral sense organ consisting of 

 a plate of thickened ectoderm provided with long stiff cilia. The 

 line joining mouth and sense organ would be the chief axis of the 

 body. The digestive tract would consist of a stomodaeum and a 

 more or less spacious sacculated enteron, and would be surrounded 

 by a mesenchymatous tissue consisting of a gelatinous matrix 

 traversed by branched muscular fibres, derived from a special germ 

 layer, the mesoblast. Such an ancestor would itself be a Coelen- 

 terate and have been derived from a Coelenterate ancestor, and 

 very probably from a form resembling the early larvae of Actinians. 

 The Ctenophora are classified as follows : — 



Class Ctenophora. 



Sub-Class 1. Tentaculata. With tentacles. 



Order 1. Cydippidea, Lesson. 



Ctenophora of spherical, cyliuilrical, or compressed form, with two 

 simple or branched tentacles retractile within tentacular sheaths. The 

 meridional and stomodaeal canals end blindly, and are not produced into a 

 peripheral canal system. 



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