Evolution of Fishes from Invertebrates 63 



Dermal plates or scales. Such structural details call for 

 special emphasis, since they suggest a parting of the way 

 amongst ancient nemerteans, that led on the one hand to the 

 soft-bodied cyclostomes, and on the other to the tubercled 

 and later to the plated or scaled fishes. While most nemer- 

 teans remain soft in their surface tissues, others — e.g., spe- 

 cies of Eunemertes, Cephalothrix, Tetrastemma and Geo- 

 nemertes — may secrete glistening oval, angular, or hook- 

 like masses of a crystalline nature, and which are mainly 

 composed of carbonate of lime {^^ : 67 ; S4'- 43° 5 35'- I045 

 ^6:^6). These cells are often in direct contact with 

 others which are glandular, and so the entire morpho- 

 logical foundation exists here for further evolution of such 

 isolated placoid granules or spines as characterize the 

 primitive genera of fishes, Thelodus and Lanarkia, of 

 Silurian and Devonian strata. From these again to the 

 placoid or ganoid scales of fishes the gradation is easy. 



Closely related to such secretions amongst the 

 metanemerteans, and originating from the middle part of 

 the invaginated epidermis that forms the substance of the 

 proboscis, are the horny stylets that project as offensive 

 organs when the proboscis is extruded (Fig. 5). These 

 arise in succession from cells or groups of cells present in the 

 mid-region of the proboscis when seen at rest. Structurally 

 they suggest a very appropriate starting point for forma- 

 tion of horny vertebrate teeth, and this in a way that will 

 be more fully explained in a later section. As described 

 by Montgomery and by Bijrger (op.cit. 227-29) they origi- 

 nate as a secretion of the modified epidermal cells of the 

 mid-proboscis, and consist of two to five distinct and con- 

 centric layers of tooth substance, the outermost of which 

 has a shining structureless aspect. 



In various genera then of the Metanemerteans two epi- 

 dermal formations arise: (i) calcareous epidermal secre- 

 tions that form alongside glandular epidermal cells; and 

 (2) the horny proboscis teeth or stylets that undergo 

 steady wear and renewal. The former, by increasing com- 

 plexity in structure and by increased activity of the related 

 epidermal cells, would start formation of the tubercles, 

 spines, and ultimately the elaborate plates and scales that 



