NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS I3 



larval body and not from the teloblasts, the first teloblastic segment 

 being that of the first maxillipeds. In the Pericarida, however, he 

 says the boundary between the primary tissue and the secondary 

 tissue is between the segment of the mandibles and that of the first 

 maxillae. 



The accounts given by Manton (1928, 1934) of the embryonic 

 process of secondary segmentation in Hemimysis and Nehalia are 

 essentially the same as those of Sollaud for the embryo of Leander 

 and of Fransemeier for the larva of Arteniia. Manton agrees with 

 Fransemeier that the teloblastic segments include both maxillary seg- 

 ments. In Hemimysis, she says, the naupliar and postnaupliar meso- 

 derms are at first some distance apart, but later the teloblastic ecto- 

 derm and mesoderm extend forward as far as the first maxillary 

 segment inclusive. The teloblasts of Nehalia are differentiated at the 

 sides of the posterior blastoporic area, and the ectodermal teloblasts 

 eventually form a complete circle around it. The mesodermal telo- 

 blasts, according to Manton, in agreement with Sollaud, are formed 

 from the mesendodermal mass at the blastopore ; Fransemeier says 

 they are proliferated from the ectodermal teloblasts. The ectodermal 

 teloblasts, according to Manton, join the naupliar ectoderm between 

 the mandibular and first maxillary segments, so that "all segments 

 between the mandibular segment and the telson are formed by the 

 teloblasts." The rudiment of each segment arises from one transverse 

 row of descendants from the original ectodermal and mesodermal 

 teloblasts. When the last abdominal segment is completed the telo- 

 blasts disappear in both Hemimysis and Nehalia. 



Since the teloblastic generation of secondary somites added to the 

 primary segmented body of the young larva or embryo is characteris- 

 tic of the annelid worms and recurs in many of the arthropods, it must 

 have been a way of lengthening the body developed in the very primi- 

 tive wormlike ancestors of the two groups. The annelids and the 

 arthropods, therefore, did not diverge until this method of growth 

 was well established. Elsewhere the writer (1938) has suggested that 

 telogenesis may have originated as a means of increasing the repro- 

 ductive function by distributing the germ cells from the zone of 

 growth through a larger number of segments. 



III. EXAMPLES OF CRUSTACEAN METAMORPHOSES 



The metamorphoses of Crustacea are so diverse that in a brief re- 

 view of the subject we can include only a few examples representative 

 of some of the principal orders. Since crustaceans that hatch at an 

 early stage of ontogeny go through anamorphic phases of development 



