Histology 



311 



Fig. 252. Types of multicellular glands. (A-D) Tubular. (E, F) Alveolar or 

 acinous. (.4) Simple. (B) Coiled. (C-F) Branched or compound. The duct pierces 

 the epithelium from which the gland has been produced. (Courtesy, Kingsley: 

 "Comparative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) 



NONEPITHELIAL TISSUES 



The primarily essential parts of a metazoan animal are the epi- 

 dermal epithelium and the enteric epithelium. Certain of the organs 

 which, in the adult, lie between these two layers consist of tissues 

 which do not retain the epithelial character of the embryonic tissues 

 from which they are derived but give rise to more or less bulky and 

 solid masses of material. 



The important types of adult nonepithelial tissues are the follow- 

 ing: (1) muscular; (2) nervous, exclusive of neuroepithelial structures; 

 (3) tissues serving for mechanical support — the connective and skeletal 

 tissues; (4) adipose tissue or fat; (5) blood. 



MUSCULAR TISSUE 



Locomotion in some protozoans is effected by beating of cilia. The 

 movements of large animals depend on contractile mechanisms. Con- 

 tractility is inherent in protoplasm. The least specialized protoplasm 

 is apparently able to contract in the direction of any of its axes. When 

 protoplasmic mechanism for effecting vigorous, quick, or long-con- 

 tinued contracting is established, the ability to contract becomes 

 restricted to one axis. The protoplasmic structures which seem to be 

 somehow immediately concerned with contraction are exceedingly fine 

 fibrils, the myofibrils, which extend through the cell parallel to the 

 axis of contraction. 



The movements of Hydra are caused by contractile fibers which 

 are not independent cells but are merely long processes from the basal 

 ends of the epithelial cells of both ectoderm and endoderm. It is sig- 

 nificant that, in the absence of a mesoderm, the two primary epi- 

 thelia can produce an essentially muscular layer situated at the mid 

 ("meso-") level of the wall of the hollow animal. 



