314 CRUSTACEA. 



larval stage consisting of few segments. In the accurate repro- 

 duction of the ancestral characters, we should expect the Crustacea 

 also to develop through such larval stages ; instead of which we 

 find the Nauplius stage as the typical starting point of Crustacean 

 metamorphosis. The larvae of the Crustacea are thus secondarily 

 modified by the premature development of Crustacean characters. 

 HATSCHEK was inclined, in accordance with the then prevailing 

 conception of the Nauplius, to compare it, as an unsegmented form, 

 with the Annelidan Troclioplwre. More recent observations have 

 shown, however, that the Nauplius possesses several true body- 

 segments. This is especially supported by the fact, proved by 

 CLAUS and DOHRN, that the second pair of limbs of the Nauplius 

 is innervated by a post-oral ganglion. The Nayplius consequently 

 has to be considered as an already segmented larval form, and 

 comparable at least with an already metamerically segmented young 

 Annelidan larva (CLAUS, No. 7). Opinions as to how many trunk- 

 segments can be attributed to the Nauplius will vary according 

 to the view taken as to the segmentation of the cephalic section 

 in the Crustacea. It appears to us best to agree with the facts of 

 ontogeny and with the anatomy of the Crustacean brain to claim 

 a true trunk-segment for each pair of the Nauplius limbs, and 

 to assume, in addition, a primary cephalic section lying in front of 

 these, and a posterior terminal region (anal segment) closely united 

 with the budding zone (for the formation of new segments), this 

 latter region giving rise to the telson (cf. on the primary segmentation 

 of the head in the Crustacea, p. 164, and on the Nauplius stage, 

 p. 191). 



The transition from the Annelida to the ancestral forms of the Crustacea 

 (Protostraca, CLAUS) was connected with certain modifications of structure 

 and of method of locomotion. Even in pelagic Annelida (e.g., Tomopteris), 

 locomotion is effected by lateral serpentine movements of the body. The 

 mobility of the segments inter se here comes to the fore, the parapodia having 

 only a slighter degree of independent mobility. The greater cuticularisation 

 of the surface of the body in the ancestors of the Crustacea, led to the 

 limitation of the mobility of the metameres inter se. The trunk became firm 

 and more rigid, while the limbs became articulated with the trunk and capable 

 of independent movement. It is certain that a more perfect form of movement 

 with less expenditure of force was thus attained. The transformation of the 

 Annelidan parapodia into independently movable oars indicates the reason for 

 the modifications in shape undergone by these appendages, which finally led to 

 the development of the biramose Crustacean limb. In view of the fact that many 

 of the parapodia of pelagic Annelida actually assume a lamellate form, we may 

 suppose that the hypothetical ancestor of the Crustacea had similarly shaped 

 limbs. We shall therefore be inclined to accept the view that the lamellate and 



