CRUSTACEA— ONTOGENY 



385 



Development of Cetochilus (Order Copepoda, Family CalanidEe). 



1st Larval Stage. Typical Nauplius (Fig. 258, ^).— At the basal joint of the 

 2d antenna there is a masticatory process characteristic of most Crustacean Nauplii. 

 The mouth is overhung by an enormous upper lip, which is also characteristic of 

 many other Nmiplius laryas. The anal aperture is not yet formed. 



2d Larval Stage.— The body, and especially the posterior division, has gi-own 

 longer. Anteriorly there is a small shell-fold. At the end of this stage the first 

 pail- of maxillEe appear as small biramose feet behind the pair of mandibular feet. 



3d Larval Stage. Metanauplius (Fig. 258, B and C).— Behind the first i^air of 

 maxillae the 2d, and behind these the 2 anterior pairs of trunk limbs, are forming. 



Fig. 258.— Larvae of Cetochilus septentrionalis (after Grobben). A, Nauplius; B, Metanau- 

 plius with the rudiments of the first 2 thoracic limbs from the side. G, older Metanauplius. 

 ol, Upper lip ; m, mouth ; g, brain ; gz, genital cells ; an, anus ; me, primitive mesoderm cells ; 

 5, the two maxillipedes of the left side (exo- and endopodite of the posterior maxillce). 



all as biramose feet. The dorsal shield covers the anterior part of the body as far as 

 and including the 2d maxiUar segment. Masticatory ridges have developed on the 

 basal joints of the maxillipedes 



Further Larval Stages. — During several moults a third trunk segment forms. 



Ist Cetochilus Stage. — The furcse at the end of the body are completed. A 4th 

 trunk segment and the 3d trunk Umbs begin to form. 



From what we find in other Oopepoda, the only parts needed to complete the form 

 of the adult animal are the remaining trunk segments and the 2 posterior trunk feet, 

 aU of which form during successive moults. The 2 pairs of maxillipedes which are 

 separately inserted on the body, and are characteristic of the adult Copepoda in 

 reality only correspond with the branches of the 2d pair of maxillae from which they 

 come, and thus strictly speaking represent but one pair of extremities. 



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