16 General Fart. 



Chelonians. This is true, also, although in a more limited sense, for the 

 endoskeleton, which usually not only supports the body but also protects 

 •certain of the organs, e.g., the skull and vertebral column of the Vertebrata 

 protecting the central nervous system. 



3. Muscular System. 



Muscular tissue occurs as a subordinate component of many 

 ■organs^ e.g., th.e skin, the alimentary canal. It is, however, the 

 principal and essential constituent of muscles, those organs which 

 cause movements of the body as a whole, of individual portions of it, 

 or of its appendages, and which, taken together, constitute the 

 muscular system. In many of the lower animals which have 

 neither exo- nor endo-skeleton, the musculature is closely adherent 

 to the skin, and forms a continuous layer beneath it. In many 

 worms, there is such a body-wall, causing movements of the 

 animal by its contractions. The formation of an exoskeleton 

 has a great influence upon the development of the musculature, 

 especially when the former is divided into a number of movable 

 pieces (as in Crustacea, etc.) ; the continuous coat is then separated 

 into a number of more or less independent strands, the muscles, 

 extending between adjacent portions of the skeleton and causing them 

 to move upon one another. The muscles are still, however, connected 

 with the skin, of which the skeleton is indeed, in this case, only a 

 part. The connection ceases, where, as in the Yertebrata, an e n d o - 

 skeleton is developed, for the muscles are now attached to this, and 

 movements of the body are chiefly caused by movements of its 

 ■different parts. 



It has already been stated that the essential part of a muscle is 

 muscular tissue, but this is not its only constituent, a certain amount 

 of connective tissue is usually present, surrounding and holding 

 together the muscle elements, and often forming tendons at the ends 

 of the organ. The tendons are thinner than the muscle proper, often 

 narrower, and are composed exclusively of fibrous connective tissue. 

 They make it possible for the contractile, thicker part of the muscle, 

 to be at a considerable distance from the spot at which the force is 

 applied. The name tendon has been used to designate not only these 

 •connective tissue organs, but also others of a like significance, but of a 

 different structure, as will be seen in the detailed account of the 

 Arthropoda. 



In the lower Metazoa movement of the body is caused at least in part, 

 by the cilia of the epidermis (or of some portion of it). This is especially the 

 <!ase in many minute free swimming lai-vse (Ccelentera, Echinoderma, Chaetopoda, 

 Mollusca), which are driven about by ciliary movements. In these fonns 

 there is an actual locomotor apparatus consistiug of cUia, which either cover the 

 whole body uniformly, or are restricted to definite lines or rings. In the adult, 

 on the other hand, the cUiated cells are but seldom of locomotor importance 

 {Platyhehninths, Rotifers). 



