n 4 SCIENCE PROGRESS. 



Ctenophores are indubitably ectodermal both in origin 

 and position ; and I shall show below that the chroma- 

 tophores of Cephalopods which are mesodermal in position 

 are ectodermal in origin. 



In the second place it can be shown that pigment-cells 

 which are usually sub-epidermal in position in certain groups 

 have in all probability migrated inwards from the ectoderm. 

 The Nemertea offer good examples of this phenomenon. 

 In the Protonemertini, e.g., in Carinella, which possess no 

 cutis, the pigmented cells form part of the ectodermal 

 epithelium (Burger, /. c, taf. ix., fig. 123); and the Meta- 

 nemertini, or Enopla, retain this primitive condition (/. c> 

 p. 178, fig. 127). But the Heteronemertini possess a sub- 

 epithelial layer or cutis which is not found in the other 

 groups, and branched pigment-cells are contained within this 

 layer. Deeply as the pigment-cells extend in many forms, 

 e.g., Linens gcsserensis, they are confined to the external 

 glandular layer of the cutis in the most primitive Hetero- 

 nemertines, e.g., Etipolia, and in Eupolia Brockii the pig- 

 ment-cells are only partially contained within the cutis, 

 their upper extremities stretching between the epithelial 

 cells to the external surface of the epithelium (Burger, 

 /. c., pp. 2>Zi 5°> fig- 2t >). The series is complete, and 

 there can be scarcely a doubt that the pigment-cells of 

 Heteronemertines are ectodermal elements which have 

 sunk beneath the epithelium pari passu with the elonga- 

 tion of the ectodermal elands. 



In certain Gephyrea pigment-cells are found within the 

 cutis of the adult ; and in the case of Sipunculus nudus, at 

 any rate, they are known to be preceded by pigmented 

 bodies in the ectoderm. 



Among Crustacea Malacostraca the chromatophores are 

 situated in the mesoderm in the higher forms. But they 

 have been observed in the ectoderm in some Isopods, and 

 just beneath it in others ; in the latter case they often 

 possess processes which stretch up through the epithelium 

 to the cuticle above it. 



Again, in Echinoderma we have seen that in certain 

 cases the pigment is contained in prolongations of the 



