312 



CRUSTACEA. 



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

 being only a secondarily acquired cliaracter. But even for these 

 last forms it lias Ijeen 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 Avere 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 plienomena 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 Mastigopus 

 stage of the Sergcstidac, 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 vie^\•, there would have 

 developed from the primitive Phyllopods, lirst I\cbalia-\i\M, so-called pre-zoaeal 

 forms, from these ZoaeaAike forms, and from the latter the post-zoaeal Mala- 

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

 in Nebctlia 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 JMalacostraca). 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 jjhylogenetic }ihenomenou. Indeed, many 

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

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

 caenogenetically. LANof considers the limbs from their first ajipearance 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 atibrded by the ontogeny of Lucifer, 

 according to BiiooKS). This may i-epresent 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 

 a change in the process of development would take p)lace especially in those 



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



t Text-book of Coinjiarative Anatomy, English Ed., Vol. i., pp. 406-410. 



