276 



HABIT AND INTELLIGENCE. 



[chap. 



NOTE. 



AXOHALIES OF DEVELOPMENT. 



Metamor- 

 phosis of 

 Sitaris. 



Develop- 

 ment of 

 Echino- 

 denns. 



Pseud- 

 embryo. 



Although I am convinced of the truth, of the principles set 

 forth in the foregoing chapter, I admit that there are facts of 

 development and metamorphosis for which I am utterly unahle 

 to suggest any -vray of accounting. One of these is that of the 

 Sitaris beetle, -which, when hatched, has six legs like a mature 

 insect : these afterwards become rudimentary, and it assumes 

 the ordinary form of a worm-like larva, and is transformed into 

 a beetle in the usual way.^ 



The development of some of the Echinodermata cannot be 

 called an exception to the ordinary laws of metamorphosis only 

 because it lies altogether outside of them.- As already stated, 

 the aquatic Invertebrata are usually developed out of a germ 

 which is nothing but a minute mass of structureless sarcode, 

 bearing cilia on its surface, by means of which it swims about. 

 Dr. Thomson calls this the pseudembryo : perhaps pre-embryo 

 would be a better word. "SMthin it the true embryo is afterwards 

 developed ; and in most cases the external ciliated " sarcode 

 layer" is absorbed by the developing embryo, and disappears. 

 The peculiarity of some Echinoderms is, that the pseudembryo 

 undergoes a development of its own, which has no morphological 

 relation to that of the future animal Its form varies greatly in 

 different species. In some it is described as " vermiform ;" in 

 the sea-urchin and star-fish it appears more like the Ciliograda 

 than any other form.^ The difference between a larva and a 



1 Stated on the authority of M. Fabre, in Darwin's Origin of Species, 

 p. 530. 



2 The facts in this paragraph are taken from Dr. Wyville Thomson's 

 papers in the jS'atural History Review for July 1863 and October 1864. 



3 Professor Sars, who discovered the Bipinnaria, or pseudembryo of the 

 star-fish, befoie he knew what it was, thought it might be allied to the 

 Ciliograda. The resemblance in external form of Dr. Thomson's figures 

 of the various Echinus or Sea-urchin pseudembryos to the Ciliograda is, 

 1 think, obvious enough. 



