ORGANS. 27 



radiate symmetry of star-fishes and sea-urchins, may occur 

 to you as an exception, but it is more apparent than real, 

 and, as their bilateral embryos show, it is secondarily de- 

 rived. Another very important fact in symmetry is the 

 formation of successive segments, as in Annelids and 

 Arthropods. In the course of our systematic survey, we 

 shall refer to other questions of symmetry, such as the 

 marked lop-sidedness in snails, and the contrast between 

 backboneless and backboned animals, according to which 

 Vertebrates may be thought of as arising from worms turned 

 upside down, from inverted Invertebrates. 



II. Organs. 



We usually give this name to any well-defined part of an 

 animal, such as limb or liver, heart or brain. The word 

 suggests the idea of a piece of mechanism, but the animal is 

 more than a complex engine, and many "organs" have 

 several different kinds of activities to which their structure 

 gives no clue. 



Differentiation and Integration. — When we review the 

 animal series, or study the development of an individual, we 

 see that organs appear gradually. The gastrula cavity — the 

 future stomach, etc., — is the first acquisition, but some would 

 make out that it was primitively a brood-chamber. To 

 begin with, it is a simple sac, but it soon becomes complic- 

 ated by digestive and other outgrowths. The progress of 

 the individual, and of the race, is from simplicity to 

 complexity. VVhen we think over the animal series we also 

 notice that before definite nervous organs appear there 

 is diffuse sensitiveness, before definite muscular organs 

 appear there is diffuse contractility, and so on. In other 

 words, the attainment of organs means specialisation of 

 parts, or concentration of functions in particular areas of 

 the body. 



Contrast a frog with Hydra, and one of the great facts 

 about the evolution of organs is illustrated. Among the 

 living units which make up a frog, there is much more 

 division of labour than there is among those of Hydra. An 

 excised representative sample of Hydra will reproduce the 



