II. Organs. 10. Beproduction . 11. The Body Oavity. 39 



Under heterogony, are included all cases of regular 

 alternation between sexual gener a ti on Sj whether these 

 differ in appearance or in their mode of propagation^ therefore alter- 

 nations of parthenogenetic and dioecious generations, as well as 

 seasonal dimorphism come under this heading. 



Heredity. — The offspring produced by an animal or by a pair 

 of animals is, generally, when fully developed, just like its parents. 

 This similarity extends not only to specific characters 

 {see Section V.), but, in great measure, to the individual 

 peculiarities of the progenitors. But such features are not always 

 inherited, some may be passed over : frequently, also, the offspring 

 exhibits a greater resemblance either to the male or to the female 

 parent ; or again, the young ones may, under the influence of the 

 environment differ in small points from the parents. 



Sometimes characters which are present in an animal do not 

 appear in its offspring, but skipping that generation, are seen again 

 in the next. An animal may thus possess certain peculiarities which 

 are present, not m the parents, but in the grandparents ; indeed, 

 characters from still further back in the line of ancestors may 

 reappear: this peculiarity is called atavism. 



The phenomena referred to above (metagenesis and heterogony) 

 are not opposed to the fundamental principles of heredity. Even if 

 the offspring is unlike the parent, and sometimes very unlike, yet it 

 always bears some resemblance to an earlier generation ; no persistent 

 deviation is produced in these cases which may perhaps all be 

 regarded as regular atavisms. 



11. The Relations of the Organs to One Another.— 

 The Body Cavity. 



The organs mentioned above form the metazoan body, and are usually 

 held together by connective tissue. This often fills up the interspaces, 

 so that the animal forms a compact mass (Platyhelminthia) ; it is 

 not the case, however, in most animals, where there is a large body- 

 cavity, in which some of the organs, viz., the digestive tract, the 

 excretory and genital organs are enclosed, being generally fastened to 

 the walls by threads, or thin sheets (mesenteries) of connective 

 tissue. 



The body cavity is often divided into compartments by septa : in 

 Mammalia, for example, by the diaphragm, into thorax and abdomen : 

 in Ghsetopoda by transverse septa into many compartments. The 

 body-cavity is usually more or less completely filled with organs 

 known as the " viscera," the remaining space containing a fluid, which 

 is sometimes blood, since the vessels are frequently in free com- 

 munication with it. Besides the body cavity, other spaces may be 

 present, of various forms, sizes, and significance, and filled with a 



