500 LIFE HISTORY STUDIES OF ANIMALS. 



defensive polyps (which may be either batteries of nettling cells or 

 covering organs); and reproductive individuals. As the individuals 

 become subordinated to the colony, and lose essential parts of the 

 primitive structure, they pass insensibly into organs. 



The life histories of invertebrates abound in complications and para- 

 doxes. Thus UucJiaris, one of the ctenophors, becomes sexually mature 

 as a larva, but only in warm weather. This happens just after hatch- 

 ing, when the animal is of microscopic size. Then the sexual organs 

 degenerate, the larva, which has already reproduced its kind, grows to 

 full size, undergoes transformation, and at length becomes sexually 

 mature a second time.^ There is often a striking difference between 

 the early stages of animals which are closely related, or a strong adapt- 

 ive resemblance between animals which are of very remote blood rela- 

 tionship. In the hydrozoa similar polyps may produce very different 

 medusae, and dissimilar polyps medusae that can hardly be distinguished. 

 Tbere are insects so like in their adult state that they can only be dis- 

 tinguished by minute characters, such as the form and arrangement of 

 the hairs on the legs, and yet the larvae may be conspicuously different.^ 

 Annelids and echinoderms yield fresh examples of the same thing. In 

 lepidoptera and sawiiies the larvae are very similar, but the winged 

 insects quite different.^ New stages may be added in one species, while 

 closely allied species remain unaffected. In Gunina and the diphyidae 

 we get combinations wjiich strain the inventive powers of naturalists 

 even to name. Natural selection seems to act upon the various stages 

 of certain life histories almost as it acts upon species. 



But the history is not always one of growing complexity. Some- 

 times, for examiDle, a well established medusa stage is dropped. First it 

 ceases to free itself, then the tentacles and marginal sense organs dis- 

 appear, then the mouth closes. In the fresh-water Cordylophora the 

 medusa is replaced by a stalked sac filled with reproductive elements 

 or embryos. The lucernariae present a single stage which seems to be 

 polyp and medusa in one. Hydra has no medusa. It is not always 

 clear whether such hydrozoa as these are primitive or reduced. Even 

 the hydroid polyp, the central stage in the normal hydrozoau life his- 

 tory, may be suppressed, and certain medusae in both of the chief 

 groups develop direct from the Q,g,g or planula [Pelagia, Geryonia, 

 ^gina, Oceania). There is no stage common to all hydrozoa except 

 the egg. The same thing may be said of the tunicates. 



The life history of many arthropods is to all appearance quite sim- 

 ple. There emerges from the egg a spider, scorpion, or centipede (in 

 most chilopoda) which merely grows bigger and bigger till it is adult. 

 But if, as in most Crustacea, the circumstances of the species call for a 



iChun Die pelagische Thierwelt, page 62 (1887). 



"Some species of Chironomus are referred to. 



■^ Baron Osten Sacken (Berl. Entom. Zeits., Bd. XXXVII, page 465) gives two cases 

 of diptera, iu which "almost similar larvte produce imagos belonging to different 

 families." 



