312 CRUSTACEA. 



segments in the middle region of the body, was thus recognised as 

 being only a secondarily acquired character. But even for these 

 last forms it has been distinctly shown by the more recent researches 

 of Claus (No. 6), that these segments are not, as was. formerly 

 assumed, altogether wanting, but that they are present, although 

 in a form difficult to recognise externally, and in a very compressed 

 condition. Thus Claus has observed that, in the Zoaea stage, all 

 the pairs of ganglia of the apparently undeveloped segments in the 

 middle of the body were already present as the closely crowded 

 thoracic ganglionic mass, already perforated by the sternal artery, 

 a relationship which foreshadows the adult condition, as does the 

 whole of the vascular system of the Zoaea. 



Balfour, however,* believed that certain phenomena of Malacostracan onto- 

 geny, especially the disappearance and reappearance of some of the appendages 

 (the mandibular palp, the last two pairs of thoracic limbs in the Mastigqpus 

 stage of the Sergestidae, the three posterior pairs of maxillipedes in the 

 Stomatopoda), could only be explained with the help of a racial form which, 

 in many respects (above all, in the rudimentary condition of the middle region 

 of the body), showed Zoaean characters. According to this view, there would have 

 developed from the primitive Phyllopods, first Ncbalia-\ik<z, so-called pre-zoaeal 

 forms, from these Zoaea-Wk.^ forms, and from the latter the post-zoaeal Mala- 

 costraca (Thoracostraca and Arthrostraca). Though it is a remarkable fact that 

 in Nebalia the eight thoracic segments are closely crowded together and 

 compose a comparatively short portion of the body, and that the limbs of this 

 part are Phyllopoda-like in appearance, we are not bound to assume that 

 this region underwent still greater reduction in the racial forms which led to 

 the primitive Schizopoda (the ancestral forms of the other Malacostraca). With 

 regard also to the disappearance and reappearance of individual appendages. 

 it should be pointed out that this feature remains equally unexplained whether 

 regarded as an ontogenetic or as a phylogenetic phenomenon. Indeed, many 

 reasons may be given Avhy this need not appear so altogether inexplicable, under 

 the assumption that we have here a process of development which is modified 

 caenogenetically. LANGf considers the limbs from their first appearance in 

 the younger larval stages to be inherited from an ancestral form (Annelida), 

 while the temporary disappearance of the same can be explained by the change 

 in the condition of life of the pelagic larva. It must also be pointed out that 

 these limbs in the adult condition are usually of quite a different shape from 

 that in which they first appear in the larva. It is repeatedly found that limbs. 

 while changing from one form to another, pass through a temporary intermediate 

 stage (striking examples of this are afforded by the ontogeny of Li'.ci/rr, 

 according to Brooks). This may represent an abbreviation of development, 

 where, instead of a gradual change taking place in a limb, an ontogenetic 

 stage has been introduced, in which, after complete loss of the larval limb, 

 the differently shaped adult limb simply appeared as a new rudiment. Such 

 • change in the process of development would take place especially in those 



* Text-book of Comparative Embryohxji/, Vol. i., p. 508, 2nd. Ed. 



f Text-book of Comparative Anaiomy,KngliBh Ed., Vol. i , pp. 406-410. 



