68 



HISTOLOGY 



Shell of Loligo Pealii. A characteristic organ of the mollusca is 

 the shell, a thick layer of chitin, usually reenforced with carbonate of 

 lime, that is developed on all, or a part, of the mantle. 



-mus. 



^ sec. ep. 



FIG. 69. Transverse section through quill (shell) of a squid, Loligo. qu., quill; sec.ep., secret- 

 ing epithelium of quill; mus., muscle of mantle wall; car., cartilage. 



In the squid this "quill" is carried inside the body by an invagination 

 of the surface on which it lies (Fig. 69). The formative epithelial cells, 

 here a simple columnar epithelium, surround this shell, which is of chitin 

 alone without any lime, and build it up from all sides. Its beautiful 

 ribbed structure makes it an ideal organ 

 of support. It is laid down in layers, 

 as are all mollusk shells. At the points 

 where it is thickest the cells that form 

 it are also longest, and form, in conse- 

 quence, the thickest epithelium. 



Another kind of rigid connective sub- 

 stance is found in the interior of the 

 body in many animals which is a mass 

 of material formed by many cells work- 

 ing jointly. The tissue is called cartilage, 

 and the substance formed by its cells 

 is composed of several organic materials. 



The cartilage found in the cephalopod mollusks shows all sides of 

 the development and structure of this tissue, and will represent the car- 

 tilages in general (Fig. 70). It begins as a loose collection of mesoderm 



FIG. 70. Developing cartilage of a 

 young squid. Mitotic figures very 

 abundant. X 1200. 



