Packard.] '^*^ [Jan. 16, 



It appears that the embryology of Limulus is scarcely more like that 

 of tracheates than Crustacea ; it is a very primitive type standing 

 nearer the branchiate arthropods than the tracheate, but on the whole 

 should be regarded as a generalized or a composite form, which with its 

 fossil allies, the Eurypterida and Trilobitoe, form a class by themselves 

 with a superficial resemblance to the Arachnida. 



It seems to us that the above-mentioned characters, which separate the 

 early embryo of Limulus from the tracheates, are as important, if not 

 much more so, than those of the absence at first of an archenteric cavity or 

 difierences in the mode of origin of the mesoblast, noted by Mr. Kingsley 

 in his brief paper on the development of Limulus. In these general, 

 primitive embryonic characters Limulus appears to be as nearly allied to 

 the annelids as to the tracheates ; and too much dependence should not, 

 it seems to us, be placed upon them in seeking to establish the true rela- 

 tions of the Palaeocarida among the arthropods. In the higher worms the 

 two longitudinal mesoblastic bands split into somatic and splanchnic 

 layers (Kowalevsky). In Mysis Mctschnikoft states that the mesoblast be- 

 comes broken up into distinct somites (Balfour's Embryology i, 436). If 

 so, then this character is not one of much importance to separate Limulus 

 from the Crustacea. The ultimate origin of Limulus from the same stock 

 as that which gave rise to the modern annelids seems not improbable. 



EXPLANATION OF THE PLATE. 



Fig. 1. — Blastodermic cuticle {hi. cut) lying upon the epiblast {ep). 

 The nuclei scattered through the latter ; the nucleolus, in these as well 

 as the mesoblast cells, consisting of a number of granules. X i A. 



Fig. 2. — Longitudinal section through an embryo before the appearance 

 of the abdominal appendages, but after the rupture of the chorion ; the sec- 

 tion passes through the six cephalic appendages (i-vi), showing the somatic 

 cavities (ms), the splanchnopleure (sp), and somatopleure [so), 1-5, the 

 indications of the five primitive uromeres ; hy, hypo or ectoblast X I A. 



Fig. 2a. — Showing the relations of the hypoblastic cells {hy) to the epi- 

 blast {ep) in the dorsal region of the embryo. 



Fig. 3. — Longitudinal section of the head and the first three appendages ; 

 TOs\ ms^, first and second somatic cavities in the preoral region of the head. 

 This figure also shows the relations of the splanchnopleure and somato- 

 pleure to the epiblast. c, large distinct cell in preoral region. X i A. 



Fig. 4. — Transverse section through the head, including the append- 

 ages, i X A. 



Fig. 5. — Transverse section through the head, showing the invagina- 

 tion, and thickening of the epiblast to form the brain ; my, myeloid sub- 

 stance of the ganglion. X i A-. 



All the longitudinal sections are from the same egg, and the transverse 

 sections from another. The figures were all drawn by the author with 

 the camera. 



