THE SKELETON. 



A. Exoskeleton. Shell or Pen. 



The shell of the Sciiiid is represented by the pen , a feather-shaped plate of 

 chitin which is buried under the skin and muscle of the anterior surface and 

 which extends from the dorsal to the ventral point of the mantle. The pen is 

 secreted by the shell-gland which is formed by an invagination of the ectoderm 

 of the dorsal surface of the very early embryo. The edges of this pit soon unite 

 with one another so that the shell-gland becomes a closed sac and the shell or 

 pen , formerly and morphologically external . becomes internal. The inner or 

 lower wall of this sac secretes the pen and is formed by a secretory epithelium 

 which at first is composed entirely of columnar cells , but Avhich , as the pen 

 enlarges, becomes cubical or even pavemental except around the edges and 

 beneath the thick ventral part of the shaft where growth is most active and 

 where thruout life high columnar epithelium is present. The chitin of the pen 

 is transparent and almost colorless and is marked by delicate striae , the lines 

 of growth , parallel to the edge. The substance of the pen yields all the usual 

 chemical reactions of chitin. 



The median axis or shaft of the pen is a trough-shaped plate which is 

 thicker ventrally and which extends the whole length of the pen. The concave 

 surface of the shaft is underneath and interlocks with a ridge of the nuchal 

 cartilage which lies in the visceral mass just behind the neck. The vane oi- 

 body of the pen is attached to the sides of the shaft, extending from its dorsal 

 end three-fourths of its length , forming a thin . fusiform . convex sheet which 

 covers the upper surface of the visceral mass. At the middle of the vane and 

 dorsal to this point , the sides of the pen extend nearly or ciuite half way around 

 the visceral mass , but ventrally the vane narrows until , at the middle of the 

 body , it covers only the anterior surface of the visceral mass. The inner surface 

 of the pen is separated from the viscera by the epithelium which secretes the 

 pen , by a strong sheet of connective tissue and by the coelomic epithelium. 

 The upper surface of the ventral end of the shaft is subcutaneous and the muscle 

 fibres of the mantle are attached to the sides of the shaft. Near the ventral 

 end of the vane , muscle fibres begin to pass over the shaft and from this point 

 backward, more and more fibres pass over the pen and fewer are attached to 



