12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



Most studies that have been made on the larval development of 

 Crustacea describe and picture the newly forming postnaupliar somites 

 and appendages as they appear externally, without giving any in- 

 formation as to how they are formed. A paper by Fransemeier 

 (^939) > however, describes the segment formation in the free-swim- 

 ming metanauplius of the branchiopod Artemia salina, and papers 

 by Sollaud (1923) and by Manton (1928, 1934) give details of the 

 corresponding segmentation in the embryos of Leander, Hemimysis, 

 and N eh alia. 



In the anterior part of the nauplius the embryonic ectoderm is 

 already differentiated into the tissues derived from it, and the meso- 

 derm has been formed from the embryonic mesoblasts. The ectoderm 

 of the body region behind the mandibles, however, is still undiffer- 

 entiated and there is here no mesoderm distinguishable at this stage. 

 At the posterior end of the body of Artemia the ectoderm forms a 

 circumanal fold, the cells of which are the ectodermal teloblasts that 

 will form the ectoderm of the new segments. From the ectodermal 

 teloblasts, according to Fransemeier, cells are given off into the in- 

 terior of the body that become the mesodermal teloblasts, which will 

 generate the secondary mesoderm. The naupliar mesoderm and the 

 postnaupliar mesoderm of Artemia are thus distinct in their origin, 

 though the formation of the second takes place 10 to 15 hours before 

 the hatching of the nauplius. The teloblasts constitute the zone of 

 growth, from which the new segments will be generated forward. 

 The first segments formed from the teloblasts are said by Fransemeier 

 to be those of the first and second maxillae. As other segments are 

 generated the anus-bearing region is carried posteriorly as a permanent 

 telson. The proliferation zone remains active until the last segment is 

 formed, when it is fully exhausted. The alimentary canal apparently 

 simply lengthens posteriorly, the proctodaeum having been formed 

 in the nauplius. 



In the young naupliar embryo of the palemonine Leander, as de- 

 scribed by Sollaud (1923), the postmandibular part of the body is 

 a small anus-bearing lobe, or caudal papilla, which subsequently 

 lengthens and projects free from the body in front of it and bends 

 forward. A transverse row of large cells becomes differentiated in 

 the ectoderm of the lobe before the anus, and later encircles the lobe. 

 These cells are the ectodermal teloblasts. Below and a little before 

 them is formed a corresponding ring of mesodermal teloblasts, which, 

 according to Sollaud, are derived from the blastopore. The teloblasts 

 generate the secondary segments in the usual manner, but in Leander, 

 Sollaud says, the two maxillary segments are formed directly in the 



