VIII.] HOMOLOGIES. 181 



and the dog before spoken of.^ For this relationship ]\Ir. 

 Eay Lankester has proposed the term " homoplasy." 



" Serial homology " is a relation of resemblance existing 

 between two or more parts placed in series one behind the 

 other in the same individual. Examples of such homo- 

 logues are tlie ribs, or joints of the backbone of a horse, or 

 the limbs of a centipede. The latter animal is a striking 

 example of serial homology. The body (exce23t at its two 

 ends) consists of a longitudinal series of similar segments. 

 Each segment supports a pair of limbs, and the appen- 

 dages of all the segments (except as before) are com- 

 pletely alike. 



A less complete case of serial homology is presented by 

 Crustacea (animals of the crab class), notably by the squilla 

 and by the common lobster. In the latter animal we 

 have a six-jointed abdomen (the so-called tail), in front of 

 Avhich is a large solid mass (the cephalo-thorax); terminated 

 anteriorly by a median process (the rostrum). On the 

 under-surface of the body we find a quantity of moveable 

 appendages. Such are, eg., feelers (Fig. 9), jaws (Figs. 

 6, 7, and 8), foot-jaws (Fig. 5), claws and legs (Figs. 

 3 and 4), beneath tlie cephalo-thorax ; and flat processes 

 (Fig. 2), called " swimmerets," beneath the so-called tail 

 or abdomen. 



Now, these various appendages are distinct and different 

 enough as we see them in the adult, but they all appear 

 in the embryo as buds of similar form and size, and the 

 thoracic limbs at first consist each of two members, as the 

 swimmerets always do. 



This shows what great differences may exist in size, in 

 form, and in function, between parts which are develop- 



^ See ante, p. 7Q. 



