656 INVERTEBEATA CHAP. 



dispersion, resulting from the existence of a free-swimming larval 

 phase, confers on it, counterbalances the disadvantage resulting from 

 the mortality of the larvae. Generally speaking, the more modified 

 and more advanced in structure a species is, the more prominent is 

 the embryonic phase in its life-history. But in isolated cases, within 

 each natural group of animals, almost the whole of the life-history 

 may become embryonic, whilst the adult structure remains compara- 

 tively unmodified. So we find that in Peripatus capensis, one of the 

 most primitive of Arthropods, the whole of the development is passed 

 within the maternal oviduct ; and that in Amphiura squamata the 

 young emerges from the genital bursa of the mother as a perfect brittle- 

 star, whilst nearly allied species have a long larval development. 

 Such cases could be multiplied, and there can usually be found some 

 reason in the peculiar local conditions of the species which makes a 

 larval life peculiarly dangerous. 



But even when larval life has begun, its even course is sometimes 

 interrupted by phases which we may term pupal, and which show a 

 striking similarity to embryonic phases. During these the growing 

 form may shelter itself in some kind of case, either of extraneous 

 matter or of its own secretions, and takes no food, whilst extensive 

 internal changes go on. In less marked pupal stages the creature 

 continues to lead a free life but takes no food. We may recall to the 

 reader's mind the pupal stages of Cirripedes and Holothuroids. 



The reason for these modifications is clear. In every larval stage, 

 as we have seen, the creature encounters dangers and has a special 

 way of obtaining food. In one and the same locality these different 

 ways are probably not equally effective. It is therefore a great 

 advantage if enough food can be accumulated during one larval stage 

 to enable the next to be passed rapidly through without needing food 

 at all. The most common form of this is to find that the larvae, when 

 just hatched, retains a store of food in its tissues from the preceding 

 embryonic stage, and whilst it moves about freely it requires no food. 

 The yolky larvae of Asterina, Cribrella, and Solaster, among brittle- 

 stars, may be mentioned, as also of most Ectoproct Polyzoa. Such 

 creatures are really intermediate in type between embryos and larvae. 



Finally, in rare cases, the alteration of conditions, either of climate 

 or of the nature of competitors, may cause the larval stage to become 

 more safe and advantageous than the adult ; and in this case the 

 adult stage is employed only for pairing and egg-laying, and is passed 

 quickly through, or it may be suppressed altogether. Originally the 

 conditions of adult life must have been more advantageous than those 

 of the larva, or the evolutionary step from the one to the other would 

 never have been taken. But we have already pointed out that the 

 imagiual stage in many of the higher insects is of very short durati< >n, 

 and that during this stage, in many cases, no food is taken ; and here 

 we must assume that the life of the larva is less dangerous than that 

 of the adult. Again, among some Urodeles, such as the Axolotl, the 

 larva can develop ripe genital organs whilst continuing the larval 



