50 OBGANIZATION AND DEVELOPMENT OF ANIMALS IN GLNEBAL. 



tissue, developed from one or both of the primary layers, primitively 

 serves as a support for the body and forms the skeleton ; and it also 

 gives rise to muscles which increase the organism's power of move- 

 ment and apply themselves, on the one hand, to the ectoderm 

 (somatic muscles), and on the other, to the endoderm (splanchnic 

 muscles). Between the primary layers of the body there is primi- 

 tively present a space, the primary body cavity.* Subsequently a 

 second, space, developed as a split in the intermediate tissue may 

 appear, giving rise to the secondary body cavity. t~ From the latter 

 the vascular system is developed. 



Contemporaneously with the appearance of muscles a nervous 

 system is usually differentiated from modified cells of the outer layer. 

 Outgrowths from the body also are developed, which may have either 

 a radiate or a bilateral arrangement. They take the form either of 

 organs of nutrition (gills) originating from the need for an increase of 

 surface, or of organs of prehension and movement (tentades, limbs). 



The increasing complexity of organization depends, therefore, not 

 only upon the extension of the surfaces endowed with vegetative 

 functions, and on the appearance of the organs of animal life, but 

 also on a progressing process of division of labour; which results in 

 a clearer and more definite localization of the various functions, 

 necessary for the maintenance of life, in special organs. The greater 

 this specialization the more completely will each organ be able to 

 discharge its special functions, and supposing a proper co-ordination 

 between the working of all the organs, a great advantage accrues 

 to the organism, which is thereby rendered capable of a higher and 

 more complete life. Therefore we find, as a general rule, that the 

 larger the body and the more complex the organization, the higher 

 and more perfect is the life. In this relation, however, the form and 

 arrangement of the organs which characterize the various groups 

 (types), as well as the special conditions of life which are limited by 

 them, must be taken into account as compensating factors. 



CORRELATION AND CONNECTION OF ORGANS. 



The organs of the animal body stand in a mutually limiting rela- 

 tion to one another, not only in their form, size, and position, but 

 also in their actions ; for since the existence of an organism depends 

 upon the blending of the individual performances of all its organs 

 to a united manifestation, the various parts and organs must all, iu 



* Usually known as segmentation cavity. ED. 



t Usually known as " body cavity," or " ccelom." En. 



