80 SOME CURIOUS FACTS CONNECTED WITH 



namely, phylogeny (or the tribal descent of animals), revealed by 

 embryology aided by palseontology, and the discovery of archaic 

 surviving forms. The other great branch of the science — also of 

 surpassing interest — deals with the modifications of organs by use 

 and disuse. 



I will first deal with the modifications of organs by use and 

 disuse. These modifications are profound, and are, perhaps, 

 more striking in animals of parasitical habits than in many others. 



Semper gives the following description of Sacadi?ia and other 

 Cirripedes, degenerate crustaceans of parasitical habits : — 



"As a rule, almost without exception, the larvae of parasites 

 move about freely in water. During this stage the larvae are 

 usually high in the scale of structure. Those of the parasitical 

 Cirripedia —iox instance, Sacculina — have, in what is known as 

 their Nauplius form, external organs of locomotion of a compli- 

 cated character, a muscular system of the Crustacean type, a well- 

 developed intestinal canal, and usually have special organs of 

 sense — eyes. Gradually, this Nauplius, after attaching itself to the 

 tail of a crab, loses its organs of locomotion, the greater part of 

 its muscular and nervous system, its organs of sense — including, 

 of course, the eye— and often its mouth, stomach, and intestinal 

 canal. The lively crab-like larva is transformed into a shapeless 

 sac, exhibiting no trace by which its crab-like nature can be 

 recognised. 



Animals adopting a sedentary mode of life tend also to lose 

 their sense organs, and especially their eyes. The free-swimming 

 Nauplius, of the sessile Cirripedia, developes six pairs of strong 

 swimming feet and a pair of composite eyes. When the Cirri- 

 pede has settled down for life, " fixing itself on its head and 

 kicking its food into its mouth," as Miss Buckley described it, it 

 has no further use for its eyes. It is glued head downwards in its 

 shell, and only the modified ends of the feet appear. 



The Ascidian larva possesses a median eye. Some Ascidians, 

 or their near relations, as we know, kept their larval tail, and, as 

 further history will show, their median eye, and developed into 

 the ancestors of vertebrate animals. Others preferred a life 

 without vicissitudes and danger, quietly anchored themselves, 

 gave up their eyes and tails, and became little else than shapeless 



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