TRANSACTIONS OF SECTION D. 679 



break off and swim away, but are Lamessed to the colony, and drag it along. Tlie 

 colony may contain feeding polyps, whicli procure and digest food for the rest; 

 swimming bells, which aru attached medusae ; perhaps a float, which is a peculiar 

 kind of swimming bell; 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, thej'^ 

 pass insensibly into organs. 



The life-histories of Invertebrates abound in complications and paradoxes. 

 Thus Eucharis, one of the Ctenophors, becomes sexually mature as a larva, but 

 only in warm weather. This happens just after hatching, 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 strikmg difference 

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

 tive resemblance between animals which are of very remote blood-relationship. 

 In the Hydrozoa similar polyps may produce very different medusae, and dissimilar 

 polyps medus£e that can hardly be distinguished. There are insects so like in 

 their adult state that they can only be distinguished by minute characters, such as the 

 form and arrangement of the hairs on the legs, and yet the larvae may be con- 

 spicuously different.' Annelids and Echinoderms yield fresh examples of the same 

 thing. In Lepidoptera and Saw-flies the larvte are very similar, but the winged 

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

 allied species remain unaffected. In Cunina and the Diphyidse we get combina- 

 tions which 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. Sometimes for 

 example a well-established medusa-stage is dropped. First it ceases to free itself, 

 then the tentacles and marginal sense-organs disappear, then the mouth closes. In 

 the fresh-water Cordylophora the medusa is replaced by a stalked sac filled with 

 reproductive elements or embryos. The Lucernarise 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 Hydrozoan life-history, may be suppressed, 

 and certain medusae in both of the chief groups develop direct from the egg 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 simple. 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 migratory stage, such a stage will be added. 

 In certain Decapod Crustacea (Penseus, Leucifer) a nauplius and as many as five 

 other stages may intervene before the final or adult stage. Some of these 

 larval stages are common to a great many Crustacea, but none, as we now 

 think, belong to the original phylogeny. If a resting or a winged stage is 

 wanted, it is supplied just as easily, witness the holometabolic insects. Here 

 again, so far as we know, there is nothing absolutely new.'' The stages which 

 seem new are merely exaggerations for special pui-poses of sections of the life- 

 history, which were originally marked out by nothing more important than 

 a change of skin and a swelling out of the body. Let us not suppose for a 

 moment that it is a law of insect-development that there should be larva, pupa, 

 and imago, or that it is a law of Crustacean development that there should be six 



' Chun, Die pelagiscJie TJderwelt, p. 62 (1887). 



^ Some species of Chironomus are referred to. 



' Baron Osten Sacken CBerl. Entom. Zeits., Bd. xxxvii. p. 465) gives two cases 

 of Diptera, in which ' almost similar larvje produce imagos belonging to different 

 families.' 



* ' Nirgends ist Neubildung, sondern nur Umbildung.' — Baer. 



