NOTES. 409 



teeth. The most extreme case in the whole series of rudimentary organs 

 which have lost their original use is offered by the roots of the Rhizo- 

 cephala (P('/frya*#w),(ng. I2,a,p. 47); the parasite plunges them into the 

 body cavity of the host it lives upon, and absorbs its nourishment 

 through them. When the parasite has reached a certain age, it falls 

 off, leaving its roots in the body of its host ; they live on, though their 

 purpose as organs of nutrition for the Peltogaxter is, of course, entirely 

 lost. 



Xote l.paye 22. This use of the clinging foot of the Geckotidae is 

 well known. In handbooks of Zoology even in Clans we read that 

 these clinging hairs or suckers are formed by the secretion of a glu- 

 tinous matter from the glands of the toes. I do not know on what this 

 assertion rests, but there is, in fact, no truth in it. In the first place, 

 there certainly are no glands present on the feet and toes ; the clinging 

 power is, on the contrary, effected in a wholly mechanical way, and 

 derived from the rows of bristles beneath the toes, resembling the 

 sucking discs of flies or of leeches. By the pressure of the foot on a 

 smooth wall, the air between it and the wall is quite driven out ; when 

 the pressure is removed, the inner surface of the foot is raised by the 

 elasticity and rigidity of the hairs, and this effect is increased by the 

 special muscles which move the lobes, each of which bears bristles ; 

 thus a vacuum is formed between the wall and the sole of the foot, and 

 atmospheric pressure secures the foothold. 



CHAPTER I. 



Nate 5, page 27. Thacker has lately attempted to solve this problem 

 on morphological grounds (Connecticut Academy, 1877, vol. iii., On 

 Median and Paired Fins, a Contribirtion to the History of Vertebrate 

 Limbs). He is of opinion that the four extremities of the vertebrata 

 are to be considered as the survivals of two longitudinal folds of the 

 skin, such as are to be seen in the living Amphioxus. If this attempt 

 could be successfully followed up, we should no longer have to seek the 

 analogues of the extremities of the Vertebrata among the Invertebrat a, 

 as Dohrn has lately done. But the physiological side of the question is 

 not touched by it ; it is this : Why could only exactly four extremities 

 be developed, and not six, eight, or more out of this skin-fold ? 

 Thacker does not go into this question. Dohrn certainly thinks he 

 has found the use of the number four as applying to the limbs ; accord- 

 ing to him a long narrow fish swims best when it possesses two pairs of 

 fins as far as possible from each other, one pair in front and one behind. 



