DEVELOPMENT OF LIMULUS POLYPHEMUS. I73 



rely upon, regarding the egg and embryonal membranes, and the formation of the blasto- 

 derm. Dohrn describes the egg of Daphnia as being transparent, with a peripheral primi- 

 tive layer (Keimschicht) surroimding the yolk, and a chorion. The primitive band 

 entirely surrounds the yolk, and the rudiments of the limbs arise as in other Crustacea. 



The mode of formation of the primitive band in Limulus was not observed, but I beheve 

 that I have given facts sufficient to show that it forms the thickened portion of a layer of 

 cells which surrounds the whole yolk. At the earliest period observed, it was, however 

 confined to a small portion of the ventral surface of the egg, forming a local thickenino- 

 of the blastoderm, resulting (as Claparede has clearly shown in the eggs of the spiders,^ 

 Pholcus, Clubione and Lycosa, where the germ forms a broad band on the ventral surface 

 of the yolk), from a multiplication of the blastodermic cells, which are much smaller than 

 elsewhere. This primitive band, according to Rathke's researches, becomes still more local 

 and circumscribed than in the spiders and scorpions, in E)-iphla sjnnifrons, Palaimon ad- 

 spersus, Crangon maculosus^ and Astacus fluviatilis ,^ and he calls it the " primitive disc" 

 {Keimscheibe). In the eggs of all other Crustacea whose development has been studied, 

 the priiuitive band surrounds the whole yolk long before the limbs bud out. and is not gath- 

 ered into a .small, ventral disc. Now all the above named ametabolous Decapoda and Amphi- 

 poda undergo no metamorphosis after leaving the egg, neither do the spiders and scorpions, 

 while the metamorphosis of Limulus is very slight. The question naturally arises whether 

 there is any connection between this origin of the embryo fi-om a primitive disc, so excep- 

 tional a case in Crustacea, and the equally exceptional ametabolous growth of the same 

 species of Crustacea ? 



While the Cladocera (Daphnia and others) escape from the egg with their full number of 

 limbs, and with the general form of the adult ; the larvae of Apus (see p. 163, the youno- 

 after the first moult) and Branchipus leave the egg in the nauplius stage, comparable with 

 the larvi:e of the Copepoda and, in a less degree, with those of the Cirripedia, and afterwards 

 pass through a well marked metamorphosis. Fritz Miiller * also states that the Phyllopoda 

 " have to undergo a metamorphosis. It seems to be more marked, however, in the genus 

 Nehalia, which Mecznikow says ' passes through the nauplius and zoea stages, which, in 

 the Decapoda, occur partly (in Pe?ioe?<s) in the free state.' Therefore I regard Nebalia as 

 a Phyllopodiform Decapod.® " 



The Phyllopoda also, without exception, so far as our knowledge extends, bear their eggs 

 about with them until the young are hatched. The eggs of the Cladocera are borne 

 in special cavities, or "brood-holes," while they are carried about in the Phyllopoda some- 

 what after the manner of the Copepoda, or are placed in ovisacs appended to the base 

 of the abdomen. The eggs are fertilized fi-om the union of the two sexes. Limulus 

 is an exception to this rule, the ova being deposited loose in the sand of the sea shore 

 between tide marks, where they are probably fertilized by the spermatozoa of the males 

 floating free in the water, and are left by the females to the mercy of the wind and waves. 



Though the embryo of Limulus is hatched with the cephalothorax and abdomen, and with 



1 Recherches sur I'fivolution des Araigne'es. 1862. Utrecht. * Fur Darwin. Leipzic, 1864, p. 56. English Translation, 



2 H. Rathke Zur Morphologie. Reistbcmerkungen aus London, 1869, p. 83. 



Taurien. 1837. 6 Quoted by Muller. Fur Darwin. 



* Rathke. Untersuchungen ueber die Bildung und Ent- 

 wickelung des Flusskrebses. 1829. 



JJKMnlUS BIST. POC. NAT. HIST. VOL. II. 44 



