o 



192 CRUSTACEA. 



to a trunk-segment, which became early specialised and lost its independence. 

 The cephalic and terminal segments stand in a certain conti-ast to the trunk- 

 segments, in so far as onlj^ the latter have rudiments of limbs. The ptosterior 

 portion of the Nau})lius body contains anteriorly a budding zone from which 

 new trunk-segments are continually being produced, while its posterior ex- 

 tremity eventually becomes the anal segment (telson) of the adult. 



The typical form of NaiqMus above described undergoes many variations in 

 detail, some of which will be described later. In most cases, at the Nauplius 

 stage, not only the above-named trunk-segments are perceptible, but also the 

 rudiments of other segments behind these. Such a stage, indicating as it does 

 a higher grade of segmentation of the body, is more correctly described by 

 Claus' name of Metanauplius. This name denotes all those larval stages 

 following upon the Naupliiis stage, which, while retaining the general characters 

 of the KciKjflius, show an advance upon it in segmentation by the possession 

 of trunk -segments bearing limbs behind the mandibular segment. Such a 

 widening of the conception of the A'^auplius appears the more admissible as 

 O. F. MuLLER founded his hypothetical genus Nauplius on a Cyclops larva with 

 four pairs of limbs, while to the younger stage with three pairs of limbs he 

 ajjplied the name Amymone. After it had been discovered that JMulleu's 

 genus belonged to the series of ontogenetic stages of Cyclops [Jurinc), the name 

 " Kau-plius " was adopted for all similarly-shaped Crustacean larvae. 



The Nauplius, in accordance with the views of Fritz Muller (No. 16) was 

 for a time regarded as a larval form of great significance (Haeckel, Dohrn, 

 No. 9), as it was thought to represent the common racial form of all the 

 Crustacea. In what way this racial form was to be connected with the lower 

 groups of animals was not so cleai'ly stated. It was, however, thought that 

 the nearest relations of the hypothetical ancestral form were to be sought among 

 the Rotifera or the Annelida. The first to oppose the prevalent view was 

 Hatschek (Xo. 15), who pointed out the resemblance in body -structure 

 existing between the Crustacea and Annelida, and urged that the common 

 ancestral form of the Crustacea must have had a body already composed- of 

 many segments, and that such a racial form could be directly deduced from 

 the Annelida. This view received great support when the structure of the 

 two pairs of excretory glands (shell- and antennal glands) was better knoMn ; 

 the homology of these glands with the segmental organs of the Annelida had 

 already been asserted by Leydig and Gegenbaur. Very gradually the following 

 view has been adopted and is now generally accepted (Dohkn, No. 11), viz.^ 

 that the Nauplius does not represent a stage in the direct ancestral line of 

 the Crustacea, but is a caenogenetic, adaptively modified larval form in which 

 the following Crustacean characters are earlj' developed : the form of the limbs, 

 the strong cuticularisation of the surface of the body, and in connection ^\ith 

 this, the development of hair-like processes, the absence of ciliated epithelia, 

 the disappearance of the coelomic sacs, and the development of a lacunar body- 

 cavity. The Nauplius thus shows typical Crustacean characters in its structure 

 and in its hi'itological constituents, while iji its body-segmentation it stands 

 at a level which can best be compared with that of a polytrochan Annelid larva. 

 The Nauplius, therefore, is a larval form which is secondarily modified by its 

 early adoption of the Ci'ustacean manner of life, but which, phylogenetically, 

 arose at a comparatively late stage. 



One of the jirincipal reasons advanced for considering the Nauplius to be the 

 racial form of the Crustacea was the common occurrence of this larval form 



