ELEMENTAR Y ANA TO MY. 



[LESS. 



FIG. 10. THE FLYING-LEMUR 

 ( Galeopitkecus), 



The continuity of the dorsal and ventral surfaces of the 

 limbs with the dorsal and ventral surfaces of the body, as also 

 the correspondence between the dorsal (and ventral) surfaces 

 of the two pairs of limbs, may be more plainly exhibited than 

 in man. Thus in many tortoises both the knee and the elbow 

 are rather turned outwards, than 

 the former forwards and the latter 

 backwards, as is also the case in 

 the Flying- Lemur (Galeopithecus) 

 amongst beasts. 



Bilateral symmetry is common to 

 man and the majority of animals. 

 It may, however, be much less com- 

 plete than in him, as in the fiat 

 fishes (such as the sole, turbot, 

 flounder, &c.), where both the eyes 

 come to be on one side of the head ; and it may be want- 

 ing altogether, not only as regards the viscera, but also as 

 regards the external form, as is the case in many Mollusks, 

 e.g. the snail. 



On the other hand, this kind of symmetry may be carried 

 to a far greater extent than it is in man, as is the case in 

 the Annulosa, where not only the external form, but the in- 

 ternal viscera also, are bilaterally symmetrical. 



An antero-posterior symmetry may be developed to a cer- 

 tain superficial extent, as in the reptile 

 AmphisbenaJ- in which it is at the first 

 glance difficult to tell the head from the tail. 

 Another form of symmetry which is en- 

 tirely absent in man is radial symmetry. 

 This symmetry is exhibited by jelly-fishes 

 and by sea-urchins, and star-fishes in their 

 adult condition parts radiating from a FIG 

 common centre resembling and corre- 

 sponding one with each other. 



Serial symmetry may be much less and much more 

 developed than we find it to be in man. Thus, in such 

 creatures as the oyster it is completely absent. In birds it 

 is less marked than in man, the dissemblances between their 

 wings and legs being more obvious, and the resemblances 

 more hidden, than between the human upper and lower limbs. 

 In contrast with this we find in forms closely allied to man 

 the resemblance to be exaggerated whence the name Ouad- 



1 From afjKpit;, for /*</, on both sides, and fluivtiv, to go. 



ii. A STAR 

 ( U raster). 



