86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



there is no intermediate region of free somites as in the Trilobita 

 and Agnostia (fig. 36 H, I). 



The adult structure of Limulus contains evidence of the presence 

 of 14 postoral somites, the last somite being behind the last gill- 

 bearing segment (fig. 47 D, XIV) ; but Iwanofif (1933) says that in 

 the embryo rudiments of three somites appear in the postbranchial 

 region, giving thus a total of 16 somites anterior to the caudal spine. 

 The caudal spine of the Xiphosurida is often called the "telson," but, 

 as shown by Schulze (1936), a comparison with the subterminal spine 

 of such trilobites as Mcsonacis and Olenellus (fig. 36 E, F, G), which 

 arises from a segment some distance from the end of the body, sug- 

 gests that the caudal spine of the xiphosurids may not be a true 

 terminal structure, and that several primitive somites beyond it may 

 have been lost. 



Studies on the embryogeny of Arachnida have not brought out any 

 distinction between primary and secondary somites, and the arachnids 

 have no postembryonic teloblastic growth. Schulze (1936), however, 

 has pointed out many features in the adult structure of the arachnids, 

 especially in the Acarina, that suggest the trilobite type of segmen- 

 tation. The area of the four primary somites, he shows, is often 

 evident as a differentiated anterior region of the prosoma, and in the 

 segmentation and body form of such acarinids as Oxypleurites there 

 may be seen a striking general resemblance to a mesonacid trilobite. 

 The arachnid prosoma contains six postacronal somites, and in this 

 respect, therefore, is intermediate between the trilobite "head" and 

 the xiphosurid prosoma. 



The embryonic development of segmentation in the Crustacea 

 has been particularly studied by Sollaud (1923) in the palaemonid 

 Leander. The germ band of Leander is at first V-shaped (fig. 38 A), 

 its two arms diverging forward on the blastoderm from a posterior 

 area of proliferation (CD) in the region of the blastopore, whence 

 also are proliferated forward two corresponding bands of mesoderm. 

 Each mesoderm band soon becomes divided into four consecutive 

 parts, which appear as four lobes on the surface (B). The germ 

 bands themselves gradually become less divergent, and finally their 

 anterior ends curve mesally and unite by a bridge between their 

 anterior lobes (C). At the same time the rudiments of three pairs 

 of appendages appear on the second, third, and fourth lobes, which 

 are respectively the first antennae (B, D, lAnt), the second antennae 

 {2 Ant), and the mandibles (Md). The first lobes (Pre) have no 

 appendages, but they give rise to the compound eyes and the optic 

 ganglia. There now appear in the ectoderm of the young embryo. 



