CH. IV] LIFE IN THE SEA 87 



adult. The eggs of the worms hatch out as trochospheres, which are 

 spherical or oval creatures possessing a very simple alimentary 

 canal, two eyespots, a tuft of sensory hairs in front, and a circlet of 

 cilia by means of which they swim. Successive joints or segments 

 of this larval body bud off and the adult formation is gradually 

 developed. Then if the parent has been a plankton ic dweller the 

 worm continues its life in that mode, but if it is a bottom-living 

 animal it soon settles dow^n to a life in the benthos. 



Observe that there are larval forms which are characteristic of 

 extensive groups of marine animals. The typical crustacean larva 

 is the nauplius, that of the mollusc the veliger, the echinoderm 

 larva is the pluteus, or some other characteristic form, and the 

 larval form of the worms is the trochosphere. Although very 

 considerable differences may be witnessed in the characters of the 

 adults of the species belonging to these assemblages of animals it 

 is nevertheless the case that the larval forms shew a very close 

 resemblance. We explain this by assuming that in the evolution 

 of the group there was an ancestral form which is now suggested 

 by the larva. The memory, so to speak, of this ancestral form has 

 been indelibly stamped on the species in that it is reproduced in 

 the development of each individual. If an organism occurs in 

 fossil form which resembles the larval form of any animal group 

 we are justified in assuming that (if such a view is not contradicted 

 by other evidence) this fossil form was related to the ancestors of 

 the group in which our larva occurs. Thus Limalus, the king-crab, 

 shews in its development a stage which is suggestive of the 

 structure of the Trilobites, and it is generally held that these 

 long extinct animals are in some way related to the king-crabs of 

 the present day. 



So far we have only considered the sexual mode of reproduction, 

 but there are other methods by means of which marine animals 

 multiply. Whole groups of animals reproduce by budding. Thus 

 the polyzoa have in addition to the sexual mode, developed an 

 asexual method of reproduction. These are animals which form 

 colonies, and the sea-mat {Flustra) with its leaf-like fronds will be 

 a familiar example of the group to any one who has looked at the 

 debris cast up by the tide on the seashore. Starting with one 

 individual produced by the sexual mode, a colony is formed by the 



