STRUCTURE OF ORGANISMS 471 



differences in the time of maturation of the two kinds of sex cells. In the 

 higher metazoans the diecious condition is prevalent and is usually 

 accompanied by a certain amount of sexual dimorphism. This involves 

 differences between the two sexes and these in part are of such a nature 

 that each sex is able more effectively to perform its part in reproduction. 

 Dimorphism is one aspect of the general phenomenon of polymorphism. 

 In some cases the male becomes degenerate and, rarely, even parasitic 

 upon the body of the female. In still other cases parthenogenesis prevails; 

 this may give way to sexual reproduction at times, but cases are known 

 among rotifers where it is the only form of reproduction and males are 

 not known to exist. 



481. Muscular System. — The first contractile structure met with was 

 the myoneme of the protozoan cell. In the sponges certain cells are set 

 aside as neuromuscular cells in which both irritability and responsiveness 

 are more highly developed than in other types of cells. In the coelen- 

 terates contractile fibers occur which are processes of epitheliomuscular 

 cells, and in some forms contractile cells themselves are found. A more 

 advanced condition is seen in the ctenophores in which muscle cells 

 derived from the mesenchyme take the place of contractile fibrils asso- 

 ciated with ectodermal cells or muscle cells derived from such cells. 

 Muscle cells have already been distinguished as involuntary, or non- 

 striated, and voluntary, or striated. Nonstriated fibers are of very wide 

 distribution and are the only type present in the lower Metazoa ; striated 

 fibers are developed in mollusks, arthropods, and chordates. 



In ctenophores, flatworms, and rotifers muscle fibers exist as isolated 

 fibers or are associated in bands or sheets. In the higher forms, the 

 voluntary muscles are in the form of definite organs known as muscles. 

 In the highest form of such an organ as it occurs in the higher vertebrates 

 may be distinguished a belly, or body, of the muscle and tendons at either 

 one or both ends (Fig. 43). The attachment from which the muscle 

 exerts its pull is called the origin; the attachment pulled upon, the iyiser- 

 tion. In the most highly developed muscles of the vertebrates the 

 muscle is not only enveloped in a connective tissue sheath but it may be 

 separated into bundles, each with its own sheath, each fiber also being 

 surrounded with connective tissue. In such a muscle the connective 

 tissue sheaths are continuous from one end to the other and are brought 

 together to form the tendons. Thus, while they do not interfere with the 

 contraction of the muscle fibers themselves, they resist overstretching of 

 the muscle and in that way serve to protect the soft, contractile fibers 

 which they surround. 



482. Nervous System. — In the more highly differentiated protozoan 

 cells strands of cytoplasm are found (Fig. 34) which conduct the effects 

 of stimulation, and there may be even structures analogous to nerve 

 centers present. The sponges possess neuromuscular cells. In the 



