310 CRUSTACEA. 



Great interest also attaches to the consideration of the causes which 

 have brought about secondary modifications in the metamorphosis 

 of the Crustacea. 



The view that the Nauplius stage corresponds to the racial form of 

 the Crustacea is associated above all with the name of Fr. Mulleb 

 (No. 16), and received considerable support from his discovery that 

 even among the Malacostraca there is a form (Penaeus) whose meta- 

 morphosis begins with a free-swimming Nauplius. After Haeckel, 

 in his Generelle Morphologie, had adopted this view, it received 

 the support of the most prominent investigators of the Crustacea 

 (Dohrn, Claus). For a long time it remained the prevailing view. 

 As to the way in which the Nauplius was to be deduced from lower 

 forms of Invertebrata, only careful conjectures were hazarded. 

 Worms devoid of segmentation, or with only a few segments, had 

 to be investigated, and from this point of view the Eotatoria or 

 simply-shaped Annelid larvae naturally came first under consideration. 



In the same way as the Nauplius was said to be the racial form 

 of all Crustacea, the Zoaea was regarded as the racial form of the 

 higher Crustacea or Malacostraca. This view was due chiefly to 

 the state of knowledge at that time concerning the structure of the 

 Brachyuran Zoaea. Starting from the view that the segments of 

 the central part of the body (the five posterior thoracic segments) 

 are only rudimentary in the Zoaea, or, as was repeatedly asserted, 

 are not present, F. Muller (No. 16) put forth the view that the 

 Malacostraca were separated from the Entomostraca by the entirely 

 different order of formation of the segments. He distinguished in 

 the body of the Malacostraca four regions, each said to consist of 

 five segments : the primitive body, the anterior body, the middle and 

 the posterior body. The primitive body is a direct derivative of the 

 Nauplius body, and yields the three anterior segments (those of 

 the first and second antennae and mandibles) and the two posterior 

 segments (those of the uropoda and of the telson). Later on the 

 more newly-acquired regions of the body appear intercalated between 

 the anterior and posterior portions of the primitive body, the 

 segments of the anterior body (maxillae and maxillipedes) being 

 formed first, next those of the posterior body (five anterior abdominal 

 segments), and finally those of the middle body (segments of the five 

 ambulatory limbs). This view was opposed as early as 1871 by 

 Claus from the study of the ontogeny of the Stomatopoda, in 

 which, as in the Phyllopoda, the different segments appear 

 successively in regular order from before backward. 



