RELATIONSHIP OF AMMOCCETES TO OSTRACODERMS 347 



secretion of the underlying cells passes, as is seen in Fig. 140, A and 

 B. This cuticle corresponds to the chitinous covering of the 

 arthropod, and like it is perforated with canaliculi, and, according to 

 Lwoff, possibly contains chitin. The epidermal cells rest on a thick 

 layer of most striking appearance (Fig. 141), for it resembles, in an 

 extraordinary degree, when examined superficially, a layer of chitin ; 

 it is called the laminated layer, and is characterized by the extreme 

 regularity of the lamina?. This appearance is due, as the observa- 

 tions of Miss Alcock show, to alternate layers of connective tissue 

 fibres arranged at right angles to each other, each fibre running a 

 straight course and possessing its own nucleus. Although the fibres 

 in each layer are packed close together, they are sufficiently apart 

 to form with the fibres of the 

 alternate layers a meshwork 

 rather than a homogeneous 

 structure, and thus the surface 

 view of this layer shows a 

 regular network of very fine 

 spaces through which nerve- 

 fibres and fluid pass. This 

 layer is easily dissolved in a 

 solution of hypochlorite of soda, 

 a fluid which dissolves chitin. 

 Any one looking at Ammo- 

 ccetes would say that the only 

 part of its skin which resembles 



chitin is this laminated layer, and therefore the only part of its 

 skin which would afford an indication of the nature of the 

 skeleton of Pteraspis is this laminated layer, which belongs to 

 the cutis, and not to the cuticle. Yet another significant peculiarity 

 of this layer is its entire disappearance at transformation. Miss 

 Alcock, in a research not yet published, has shown that this layer 

 is completely broken up and absorbed at transformation; the cutis 

 of Petromyzon is formed entirely anew, and no longer presents any 

 regular laminated character, but resembles rather the sub-epidermal 

 connective tissue layer of the skin of higher vertebrates. This 

 laminated layer, then, just like the muco-cartilage, shows, by its 

 complete disappearance at transformation, its ancestral character. 

 Very suggestive is the arrangement of the different skeletal 



A B 



Fig. 140. — Epithelial Cells op Ammo- 

 ccetes to show the canaliculi in the 

 Thick Cuticle (B). A, Transverse 

 Section through the Cuticle. 



