Dkcembek 21, 1885.] 



SCIENCE. 



;9i 



apods, such as Penaeus, are the most primitive. So 

 far, the phylogeny of the decapods may be regarded 

 as definitely settled, and Boas proposes no modifica- 

 tion of the accepted view; but his opinion regarding 

 the origin of the swimming decapods from the lower 

 Crustacea is novel, and the evidence which he fur- 

 nishes seems to be conclusive. The Decapoda are 

 generally regarded as the modified descendants of the 

 schizopods; but Boas points out that the order Schiz- 

 opoda is not a natural group, siuce the animals which 

 have been included in it belong to two widely sepa- 

 rated orders. 



According to this author, the Euphausiacea and tlie 

 Mysidacea are not at all intimately related. The latter 

 are not in the line which leads to the Decapoda, and 

 there is no natural group Schizopoda. He therefore 

 divides the group into two orders, — the Euphausiacea 

 and the Mysidacea : the former including the primitive 

 nnspecialized forms through which the Decapoda have 

 been evolved from the lower Crustacea; and the latter 

 containing highly specialized forms, which have been 

 evolved from the Euphausiacea along an independent 

 line, and which have no direct relationship to the 

 Decapoda. He holds, that the Euphausiacea are a syn- 

 thetic group of Crustacea which has given rise to 

 several divergent groups of descendants. Of these, 

 the decapod stem has undergone the least modification. 

 A second stem, the Mysidacea, has diverged in an 

 entirely different direction, has departed very widely 

 from the primitive form, .and has, in its turn, given 

 rise to the Cumacea, and through these to the amphi- 

 pods and isopods, the Latter being the most highly 

 modified of the Malacostraca. A third line of descent 

 from the Euphausiacea has given rise to the Squil- 

 lacea. 



The recognition by Boas of the fact, that the group 

 Schizopoda is not a natural one, and the discovery 

 that the animals which have been thus associated 

 may be divided into a very primitive group, the 

 Euphausiacea, and a highly specialized group, the 

 Mysidacea, seems to be a very great advance in crus- 

 tacean morphology. 



He gives the following definition of the Euphau- 

 siacea: — 



Malacostraca, with the mid-body and abdomen com- 

 pressed, with a well-marked bend in the abdomen; 

 carapace well developed ; the last segment of the mid- 

 body a complete ring; eyes stalked; antenna with a 

 large scale; mandible simple; first maxilla with broad, 

 one-jointed palp, and with well-developed exopodite; 

 second maxilla with asiinilarpalp, and with exopodite, 

 and a cleft lacinia interna. The appendages of the 

 mid-body or cormopods all have a well-developed ex- 

 opodite, and an epipoditc which is subdivided in all 

 except the first pair, where it is simple. The endopo- 

 dite is thin and weak, and it does not end in a sharp 

 point: it is more or less rudimentary on the last two 

 pairs. The first cormopods are not specialized as 

 maxillipeds, but are like tlie others. The abdominal 

 feet are powerful swimming-organs, with an appendix 

 interna. In the male the first or most anterior ones 

 are specialized as copulatory organs. The tail-fins 

 are well developed. The liver is composed of a great 



number of small lobes. The heart is short and wide. 

 The h.alves of the reproductive organ are united by 

 a transverse unpaired portion. Spcrmatophores are 

 present, and the spermatozoa are simple round cells. 

 There is an antennary gland. The young leaves the 

 egg as a free-swimming nauplius, and the carapace of 

 the older larva is a great phyllopod-like mantle. 



It is easy to trace the relationship between this 

 group an<l the decapods, on the one side, and, on the 

 other side, through Nebalia, to the phyllopods and 

 lower Crustacea. 



The Decapoda natantia resemble the Euphausiacea 

 in many conspicuous and highly important particu- 

 lars. In these two groups alone, among the Mala- 

 costnaca, we have a free-swimming nauplius; and in 

 both the carapace of the larva is a great mantle. The 

 abdomen is bent in both, and the integument is 

 horny. The carapace, the abdominal appendages, 

 the large tail-fin, and the pointed telson, are alike in 

 both. The endopodite of the first pleopod is a copu- 

 latory organ in the decapods as well as in the Eu- 

 phausiacea; and spcrmatophores are almost universal 

 in these two groups, while they are found in no 

 other Malacostraca. 



The close relationship between these two groups 

 can hardly be questioned ; nor is it diflScult to shoir 

 that the Euphausiacea are the primitive, and the Dec- 

 apoda the derived, form. In the presence of simple 

 epipodites, and of a four-jointed palp on the first max- 

 illa, the Penaeadae are nearer to the phyllopods than 

 Euphausia; but in all other respects Euphausia is the 

 most primitive, and it shows its close relationship to 

 the lower Crustacea by many characteristics, among 

 which are the following. The terminal joint of the 

 cormopods is. rounded and blunt, as it is in Nebalia, 

 instead of being pointed, as it is in all the Malacostraca 

 except Nebalia. There are no specialized maxilli- 

 peds; but the first cormopod is like all the others, as 

 it is in Nebalia, and all the cormopods are furnished 

 with exopodite and epipodite: while in all other 

 Malacostraca there are true m.axillipeds; and either 

 the exopodites or the endopodites, or both, are absent 

 on some or on all the cormopods. The antenna has 

 a well-developed exopodite; and in the young this is 

 flabellum-like, and very similar to that of the adult 

 Limnadia or Estheria. This feature of resemblance 

 to the lower Crustacea is shared by the young of the 

 Decapoda natantia. The first maxilla h.as a large 

 exopodite; while this is rudimentary in the Decapoda 

 and Mysidacea, the only other Malacostraca where it 

 occurs at all. The pleopods are much like those of 

 Nebalia: they are elficient swimming-organs, and 

 they are provided with an appendix interna. The 

 spermatozoa, like those of the phyllopods, are simple 

 round cells without tails; and this is true of no 

 other Malacostraca except the squillas. 



While the Euphausiacea are thus seen to be very 

 much like the phyllopods in so many important 

 features, they are true Malacostraca; and they have 

 deviated greatly from their pliyllopod ancestor, and 

 have acquired a small carapace, differentiated cor- 

 mopods with long slender endopodite, small exopodite 

 divided into shaft and fl.abellum, and an epipodite 



