334 Proceedings of the Royal Irish Academy. 



between the Malacostraca and the Phyllopoda. Some writers, like 

 Clans ('72), have laid stress on their affinity with the former group, 

 others, like Sars ('87), on their relationship with the latter. Most 

 zoolrogists, regarding the Phyllopods with their very extended seg- 

 mentation as the most primitive of all Crustacea, see in the Lepto- 

 straca, transition-forms between the Phyllopods and the Malacostraca. 

 But if this isolated and very ancient group — with representatives like 

 Hymenocaris and Ceratiocaris going back into the Cambrian Period — 

 combines, in many respects, the characters of the Phyllopoda and the 

 Malacostraca, is it not more natural to regard it as a direct offshoot — 

 modified, of course, in some particulars — of the common Crustacean 

 stock whence both Phyllopoda and Malacostraca sprang? 



In Nebalia and its few allied genera, which alone represent the 

 Leptostraca at the present day, we find a head bearing stalked eyes 

 and the usual five pairs of appendages, a thorax with eight pairs of 

 simple limbs, with lamelliform exopodites and jointed endopodites, 

 and an abdomen with eight segments, whereof the first six bear paii'ed 

 pleopods, while the eighth (anal segment) is provided with two furcal 

 processes. Now a comparison between the head-appendages of Nebalia 

 and those of Apus or Branchipus shows that the former retains de- 

 cidedly the more primitive characters. In Nebalia the feelers of both 

 pairs are elongate and normal ; in the Branchipoda they are greatly 

 reduced. The mandible in Nebalia, with its long endopodite (palp), 

 is among the least specialised of all Crustacean mandibles. From it 

 the Malacostracan mandible with its reduced palp, or the Branchio- 

 podan with no palp, could be both readily derived, while it most cer- 

 tainly could never have arisen from the last-named type. So also 

 with the maxillae : in Nebalia those of both pairs have jointed endo- 

 podites, while in Apus those of the first pair are small limbless 

 masticating plates, those of the second vestigial with hardly recognis- 

 able parts. Passing to the thoracic limbs, we find in those of JS'ebalia, 

 the protopodite with the three segments, which Hansen shows to be 

 the primitive number, a naiTow segmented endopodite and a broad 

 branchial exopodite ; in Paranebalia the expedite is slender and fringed. 

 From such a limb as this can be derived on the one hand, through the 

 Schizopoda, the legs of the specialised Malacostraca, on the other the 

 complex lamellate appendages of Apus ; while it would be hard to 

 imagine how the limb of Nebalia could have arisen from the Bran- 

 chiopodan limb. The Leptostraca are specialised in the great develop- 

 ment of the carapace ; this probably accounts for the small size and 

 crowded arrangement of the thoracic segments, which, nevertheless, 



