INTRODUCTION 21 



functions. Ill higher forms, however, the body is composed of an 

 organized system of cells wherein we see a division of labor, some 

 of the cells being digestive, others muscular, otliers serving to con- 

 duct nervous impulses, etc. 



In the great majority of marine animals the eggs or young 

 larvte are cast out into the water, and it is interesting to ob- 

 serve that even sucii sedentary forms as clams, oysters, starfishes, 

 sea anemones, corals, etc., are nearly all free-swimming in their 

 early life. 



It has long been known to naturalists that, in their develop- 

 ment, animals pass through stages which recall the adult states of 

 their more simply organized ancestors of the remote past, and when 

 it was discovered that the vast majority of marine larvaj are free- 

 swimming, it seemed probable that the most ancient marine ani- 

 mals were so, and that the sedentary habits of life seen in sponges, 

 corals, mollusks, etc., were of comparatively recent origin. We must 

 ]iot forget, however, that the free-swimming habits of the young are 

 of immense advantage in leading to a wide distriljution of animals, 

 and that it may therefore have been maintained through the agency 

 of natural selection at all times, or have been brought about as a 

 result of adaptation. 



Tlie stages passed through in the development of all animals 

 are so complex, and yet show such a striking similarity of plan 

 that tliere can be no doubt of the blood relationship of all forms one 

 to another. 



As this is a most interesting and important matter, we will 

 devote some space to its consideration. 



We will first speak of a typical case of develojDment, such as 

 is seen in the common starfish, and will then consider the more 

 complex conditions exhibited in the worms, crustaceans and 

 mollnsks. 



The immature eggs of the starfish resemble a minute sphere, in 

 the centre of which we find a small round body called the nucleus. 



When the egg is cast out into tlie water this nucleus divides 

 into two nuclei, both exactly like the original nucleus. One of 

 these new nuclei is, however, soon detached from the egg as a minute 

 ball, and is thrown out into the water. The nucleus which still 

 remains in the egg then divides into two half-nuclei, and one of 



