v CRUSTACEA PHYLOGENY 407 



The proof of the homology of the antennal and shell glands of the oviducts and 

 rasa deferentia with Annulate nephridia would also be of the greatest importance. 



Among the now living Crustacea there are two orders of the Entomostraca, viz. , 

 the Pltijllopoda (the Bntiichiopoda especially) and the Copepoda, whose organisation 

 best recalls that of the racial form, the former in the rich homonomous segmentation 

 of the trunk and the structure of the nervous system and the heart ; the latter in 

 the form of the limbs, especially the oral limbs, which still most clearly show their 

 original biramose character. The Clndocera and perhaps the Ostracoda might be 

 derived from animals like the Branchiopoda by the shortening of the body and the 

 reduction of the number of segments. The ancestors of the Cirripcdes are probably 

 nearly related to the immediate progenitors of the Copepoda. 



At first sight the results of research in comparatire ontogeny do not seem to 

 harmonise with the view just stated. Out of the egg of the Entomostraca and many 

 Malacostraca the unsegmented Nauplius larva is hatched with only 3 pairs of 

 extremities, without a heart, and without a segmented ventral chord. On account 

 of this the Crustacean racial form was formerly universally held to be a Ncn/plius- 

 like animal. It was assumed that from this racial form the Crustaceans of to-day 

 developed phylogenetically through a series of gradual transformations, in a manner 

 similar to that in which they develop ontogenetically at the present time from the 

 Nauplius by a series of metamorphoses. 



We consider these views incorrect, both for general and special reasons. 



General Reasons. (1) We know of no animal form which in an adult sexually 

 mature condition resembles a Nauplius. (2) We are not justified, without further 

 proof, in concluding that the early larval stages of an animal form closely resemble 

 the ancestors of that form. 



Special Reasons. The assumption of a racial form like a Nauplius leaves the 

 problem of the rise of the typical Crustacean organisation, of the segmentation of the 

 body, the segmented ventral chord, and the segmented dorsal heart quite unexplained. 

 It must also be emphasised that the ducts of the sexual organs, except in one single 

 case, emerge in regions of the body which in the Nauplius are not at all developed. 



On the other hand, the Nauplius, as characteristic larval, not racial form of 

 the Crustacea, is explained without difficulty by the assumption of a Crustacean 

 racial form resembling the Annelida. Just as the racial form of the Crustacea is to 

 be traced back to the Annelida, so is the larval form of the Crustacea to the larval 

 form of the Annelida. We have already mentioned the tendency in the animal 

 kingdom to shift back to earlier stages of development the characteristics of the adult 

 animal. Hence in the Crustacea, Crustacean characteristics (such as limbs and dorsal 

 shield) appear as early as in the larva which corresponds with the Troehophora of the 

 Annelida. The method of moving by means of the limbs makes the old method 

 needless, so that the ciliated circles of the Troehophora larva are no longer produced. 



In all Crustacea the 3 pairs of Nauplius limbs become the 3 most anterior extrem- 

 ities of the adult animal (2 pairs of antennae and the mandibles). The fact that, in 

 the Nauplius, just these pairs of limbs appear first can again be explained without 

 difficulty. As in the Annelida so in the Crustacea also, the body with all its organs 

 becomes differentiated from before backward. At the posterior end of the body a 

 growing or formative zone continually constricts off new segments anteriorly. It is 

 thus evident that the most anterior extremities must appear first. But why just the 

 3 most anterior, and neither more nor less ? A natural answer may perhaps also In- 

 found to this question. In a young larva which, like the Nauplius, is hatched early 

 from the egg, only a few of the organs most necessary for independent life and inde- 

 pendent acquisition of food can be developed. The 3 most anterior pairs of limbs 

 which serve for swimming may be described as such most necessary organs. The 3d 



