DEVELOPMENT OF LIMULUS POLYPHEMUS. 



183 



duce the subject here, had not an eminent carcinologist, Mr. Spence Bate, remarked that 

 "the young of Trilohites are of the nauplius form."^ We would contend that while the 

 larva of Limulus probably passes through a nauplius form early in the life of the embryo, at 

 a little later period it appears with its cephalothorax divided into distinct segments bearing 

 the full number of limbs, and with a nine-jointed abdomen, bearing three pairs of limbs and 

 ending in a broad expansion. It remains in the egg state some time, and is hatched with- 

 out undergoing the complicated metamorphoses of the Phyllopoda, and other Crustacea 

 born with a true nauplius form, such as is presented by that of Penseus. 



Compare with our figure of the recently hatched Limulus (pi. iv, fig. 13, 13 a, 13 6, 13 c, 

 13 d, and 32), Barrande's larva of Trhmchus ornatus Sternb. (fig. 33, and natural size ; en- 

 larged from Barrande's figure for the sake of easier comparison). One will see at a glance 



Q 



/0^, 



Firr 32. 



Fi<r. 33. 



Fig. 34. 



Fig. 35. 



that the young Trilobite, born without any "thoracic" segments, and with the cephalo- 

 thorax articulated with the abdomen, closely resembles the young Limulus. In both, as in 

 no other young Crustacea, the body, especially the cephalothorax, is trilobitic. In Limulus 

 no new segments are added after birth ; in the Trilobites the numerous thoracic segments 

 are added during successive moults, as Barrande's beautiful plates so clearly show. The 

 Trilobites thus pass through a well marked metamorphosis, though by no means so re- 

 markable as that of the Decapodous Crustacea and the Phyllopoda. Again, the resemblance 

 is seen in the larval Agnostus nudus Beyr. (fig. 35, also enlarged ; and natural size ; the 

 left hand figure represents the adult), where the body consists, as in Trinucleus, of the 

 cephalothorax and abdomen (pygidium), the two thoracic segments being added at a subse- 

 quent moult. Agnostus bibuUatus Barr., A. rex Barr., and A. integer Beyr., have simi- 

 lar larval forms. 



In the larva of Sao hirsufa Barr. (fig. 34, natural size, and enlarged from Barrande's 

 figui-e) neither the "thorax" nor abdomen are differentiated, and the body is orl)icular, 

 but with the cardiac region well developed, and with indications of segments which are 

 in the next moult abdominal in their position, indicating that the larva of this Trilobite 

 was not much unlike those of the others enumerated above. We feel justified, then, 

 in considering the larval Trilobite as a subzoea, and would suggest that at this period 

 it was perhaps provided with cephalothoracic appendages, while broad lamellate, swim- 

 ming and respiratory feet were very prol^ably attached to the pygidium. 



Judging from Barrande's descriptions of the eggs of the Trilobites, they must have 



1 See Facts for Darwin, p. 9G. 



