36 SCIENCE PROGRESS. 



of the system of appendages in Apus. They commence 

 anteriorly, one well developed pair on each segment. But 

 from before backward they progressively diminish in size, and 

 about half-wav along' the trunk commence to be more and 

 more crowded. The most posterior pairs are quite micro- 

 scopical, and so crowded are they that as many as six pairs 

 appear to occur on one and the same body segment. There 

 are occasionally as many as sixty to sixty-five pairs of ap- 

 pendages in all ! This phenomenon is quite unique among 

 existing Arthropods, and had long appeared inexplicable. 

 The explanation offered by the present writer was briefly 

 as follows : — 



Apus, as the most primitive of all extant Crustaceans, 

 had for its immediate ancestors animals with some sixty to 

 seventy developed segments. In Apus these sixty or more 

 inherited segments commence to appear, but the bulk of 

 them remain rudimentary. We thus have the most de- 

 veloped limbs anteriorly, while posteriorly they decrease 

 in size until they are little more than closely crowded 

 limb-buds. Each pair of limbs, therefore, represents a 

 true segment. In the posterior trunk region of Apus, 

 where more than one pair of limbs occurs on a body 

 ring, this latter is not a true segment, but a secondarily 

 formed segment, consisting morphologically of as many 

 true segments as there are pairs of limbs developing from it. 

 This explanation was strongly confirmed by an exa- 

 mination of the internal organisation of Apus, in which 

 the progressive rudimentation from front to back which 

 it postulated could be clearly traced, notably in the arrange- 

 ment of the musculature in the heart, and in the genital glands. 

 On the other hand, that each limb-bud represented a true 

 segment seemed to be established by the presence of a dis- 

 tinct ventral ganglion for each pair. Lastly, an appeal to 

 the developmental history left little doubt as to the cor- 

 rectness of our interpretation of this long-standing enigma. 

 Allowing for special modifications of the anterior func- 

 tional appendages of the Naup/ius, it is, at any stage, 

 merely a small Apus showing the same phenomenon. The 

 anterior functional limbs are followed by a series of 



