INTRODUCTION TO THE METAZOA 65 



More frequently, the plasmic products are intercellular. 

 In such instances, the cells of a connective tissue are embedded in 

 a non-protoplasmic matrix the specific nature of which may be 

 highly variable. In its simplest condition, this matrix is com- 

 posed of a gelatin-like mass within which the cells are embedded 

 to form a homogeneous or gelatinous connective tissue (Fig. 40). 

 Frequently fibrillae are formed within the gelatin (Fig. 41) 

 giving greater firmness. Depending upon the arrangement of 

 these fibrillae, their composition, and composition of the matrix, 

 several kinds of connective tissue are recognizable, as, for 

 example, fibrous connective tissues, elastic tissue, cartilage, etc. 



Muscle Tissue 



Movement is an inherent property of all protoplasm. Even 

 in the Protozoa, portions of the cytoplasm frequently become 

 altered as fibers for movement more effective than that which 



Fig. 42. — Muscle fibers from subumbrella of jellyfish, Carmarina hastaia. 

 {After Schneider, courtesy of Gu&tav Fischer). 



characterizes undifferentiated, viscous protoplasm. These par- 

 tial specializations of the protozoan cell for movement are termed 

 myonemes. Partial differentiation of cells for contraction arc 

 not infrequent in the Metazoa. The inner margins of the ecto- 

 derm cells of Hydra contain contractile threads within the cyto- 

 plasm, and consequently the covering cells of the Hydra are 

 frequently termed epithelio-muscular cells. In the nematodes, 

 the mesoderm cells forming the innermost layer of the body wall 

 undergo a partial differentiation to form a series of longitudinal 

 muscle fibers. The peripheral parts of these mesoderm cells 

 become specialized for contraction while an undifferentiated 

 mass of cytoplasm containing the nucleus protrudes into the body 

 cavity. 



Frequently, mesenchyme cells become elongated or spindle- 

 shaped and develop myofibrils within their cytoplasm. Such 

 cells when grouped together form the commonest type of smooth 

 muscle tissue. 



