492 HEMICHORDA 



that line of development after a metamorphosis. This larval 

 form, which swims freely in the sea, often in great abundance, 

 is called a Tornaria, and it is peculiarly interesting in showing 

 some apparent resemblance to the larvae of Echinoderms. 



There are some facts of much interest in the develop- 

 ment of Enteropneusts. (i) They are represented in practic- 

 ally all seas from Greenland to New Zealand. Moreover, 

 Professor Benham has reported finding a species off New Zea- 

 land which seems practically the same as a Japanese form. 

 This widespread cosmopolitan distribution is characteristic of 

 archaic types. 



(2) Some species occur in shallow water near shore, e.g., 

 Ptychodera sarniensis from the Channel Islands ; most species 

 may be called littoral ; yet Glandiceps abyssicola was dredged 

 from a depth of 2000 fathoms in the Atlantic Ocean. There 

 do not seem to be marked differences of structure correspond- 

 ing to the great differences in habitat, but it should be noted 

 that the habits (of burrowing, engulfing sand, and so on) seem 

 to be very much the same throughout. 



As an instance of the evolution-principles which it is one of 

 the aims of this book to illustrate, we may refer to a well- 

 known Enteropneust, called Ptychodera flava, Eschscholtz. 

 This is a species, i.e., a group of similar individuals, " breeding 

 true," and differing from related species in characters deemed 

 important enough to deserve a special name. Now it has been 

 shown by Punnett (1903) that in the group of animals col- 

 lectively designated Pt. flava there are to be found different 

 positions of organic stability, to which it may be convenient to 

 give "a local habitation and a name". They may be 

 "varieties" within the species. Furthermore, the greatest 

 authority on Enteropneusts, Professor J. W. Spengel of Giessen, 

 finds (1903, 1904) that Pt. flava from Funafuti is different from 

 Pt. flava from New Caledonia or from Laysan. He proposes 

 to give these " provisional " specific names : Ptychodera flava 

 funafutica, Pt. fl. caledoniensis, and Pt. fl. laysanica ; and he 

 points out that while Funafuti (8° 30' S.) lies between New 

 Caledonia (20 S.) and Laysan (26 N.) Pt.fl.fimafutica is not 

 intermediate in structure between the other two forms. Now 

 the interest of these "dry details" is great — they illustrate the 

 tendency that both animals and plants have to break up into 



