xviii SUMMARY 655 



Even the reduced size of the larva and the tendency for adult 

 organs to develop precociously will alter its relation to the environ- 

 ment. The enormously long ciliated arms of the Ophiopluteus larva 

 are necessary to sustain the growing weight of adult calcareous plates 

 which make their appearance in the later stages of larval life. The 

 free-swimming ancestor represented by the larva had almost certainly 

 no calcareous plates at all. 



When, as in the case of insects among Arthropoda, and Unionidae 

 amongst Mollusca, the larvae take to special modes of life, the modi- 

 fications of the recapitulatory history become profound, and special 

 larval organs are produced ; such as the trachea! gills of the nymphs 

 of Ephemeridae, or the hooked apices of the shell-valves in the 

 Glochidium larva, which never existed as organs in any adult 

 ancestor, and whose presence may be regarded as a falsification of 

 embryonic history. 



If it be asked how these secondary characters are to be dis- 

 criminated from primary larval characters, the answer is that primary 

 larval characters connect different groups of the animal kingdom, 

 such as the organs of the Trochophore found in almost identical form 

 in Mollusca, Annelida, and Polyzoa ; or the Nauplius larva found in 

 Phyllopoda, Copepoda, Cirripedia, and Malacostraca. Secondary larval 

 characters are confined to smaller groups, and sometimes betray their 

 secondary character by their peculiar structure. Thus the tracheal 

 gills, alluded to above, allow the oxygen dissolved in the circumambient 

 water to come into proximity, not with blood contained in these gills as 

 in all other gills, but with air contained in them, from which in turn 

 the blood derives its oxygen. Here is the clearest indication that in 

 the ancestral stock the larva was originally air-breathing in habit 

 and took to water as a shelter. 



But we have learnt to recognize yet another modifying factor in 

 development. As the race progresses in evolution and successively 

 seeks new environments, there will be a tendency to leave behind it a 

 trail of larval stages representing past conditions of the stock. In 

 some cases, as in the life-history of Penaeus, at least four successive 

 larval stages can be recognized. But in each larval stage the species 

 is exposed to special dangers and suffers enormous mortality. If 

 therefore the earlier larval stages can be passed through either within 

 the mother's body or inside an egg-shell, this mortality will be greatly 

 lessened. 



This change has taken place to some extent in all life-histories, 

 for every life-history starts with an embryonic phase. In some life- 

 histories almost the whole of the development takes place under 

 shelter, and the young animal steps out into the world ready to take 

 up the adult mode of life. Such life-histories are said to be of the 

 embryonic type. They are so advantageous from the point of view 

 of infant mortality that the wonder is that any life-histories remain 

 in which the larval phase is predominant. 



We must believe that the advantage to the race which the wide 



