DEVELOPMENT OF LIMULUS POLYPHEMUS. 



189 



Suborder 2. Etjrypterida. Cephalic shield small, flattened, subquadrate. Abdomen 

 elongated, 12-13 jointed, ending in a spatulate telson, or stout spine. Antennas partially 

 ambulatory and simple, or large and chelate ; hind pair of cephalic appendages paddle-like. 

 Metastoma forming a broad plate. Family 1. Eurtjpteridce (Pterygotus, Slimonia, Stylo- 

 neurus, Eurypterus), and family ? 2. probably including Bunodes and Hemiaspis. 



3d order. Teilobita {Paleades Dalraan). Body with a thick, dense integument, vari- 

 ously ornamented with tubercles and spines ; distinctly trilobate ; more specialized than in 

 the Merostomata, being divided into a true head consisting of six segments, covered by a 

 broad, cephalic shield ; with 2-26 distinct thoracic segments (probably bearing ambulatory 

 feet), and several (greatest number 28) coalesced abdominal segments, forming a solid 

 pygidium, sometimes ending in a spine, and probably carrying three or more pairs of 

 swimming, respiratory feet.^ Metamorphosis slight. The young subzoeseform ; being 



the body of the larva of Limulus and the adult trilobites. 

 la Cyclus Harknessi the body is deeply furrowed vertically, 



Fig. 35. 



while in C. Jonesianus the posterior two-thirds of the body is 

 smooth, divided by the cardiac ridge into two elytra-like 

 areas, while anteriorly the surface of the shield is divided 

 into from six (in some species) to eight (in C. Jonesianus) 

 lobes. The margin of the body ends behind in some species 

 in a short, small, subacute point. There are no indications 

 of limbs, and very doubtfully of eyes. 



Such is a brief description of these problematical remains, 

 in regard to which Mr. Woodward says, in 1S68, "We must 

 differ from M. de Konink in referring this form to the Trilo- 

 bites. If truly an adult, it must be placed near to Apus, 

 with the other shield-bearing Phyllopods; if a larval form, it 

 may have been the early stage of Prestwiehia, or some other 

 Limuloid of the coal-measures. Nor do we think it in the 

 least probable that the shield of Cyclus radialis was flexible 

 or contractile, its original segments being completely sol- 

 dered together into one piece;" and in 1870 he adds that 

 firom the recent investigations of Dr. Lockwood and myself, 

 " these forms may indeed be the larval stages of Prestwiehia, 

 Belinurus, etc., the antetypes in Carboniferous times of the 

 modern king crab." Were it not for the large size of these 

 fossils, some (C. Harknessi) measuring five lines in length, 

 thre'e and one-half lines in breadth, and three lines in height, 

 we sh(juld be disposed to agree with Mr. Woodward; but 

 from what is known of the size and form of the freshly 

 hatched larvje of Limulus and the Trilobites, I should infer 

 that they were either the larvae of some unknown genus 

 of LimuUda;, or adult, but embryonic, forms. The larvae 

 of Bellinurus and its allies, Prestwiehia and Euprobps, 

 were, in all probability, closely allied in their form and 

 size at the time of hatching to the larva of Limulus. But 



on comparing the deep, hemispherical form of Cyclus, with 

 the surface of the body deeply lobed over a more or less 

 extent, with the embryo of Limulus before it is hatched 

 (pi. IV, fig. 18, 18 a), we find a striking simil.irity. In- 

 deed we seem to be dealing with a distinct, embryonic 

 type of Limulidae. In Cyclus we have, in a late larval 

 or possibly adult condition, that state of Limulus in which 

 the body is deeply hemispherical, and the abdomen has just 

 been differentiated from the rest of the body, while the 

 deep, transverse lobes of the yolk are not yet absorbed, 

 as seen in pi. iv, fig. 18, 18a, in the embryo of Limulus; 

 the cardiac or median lobe being as distinctly marked in 

 Cyclus as in the embryo of Limulus. 



We may regard Cyclus as a subzoea, but in its spheri- 

 cal form much nearer the nauplius state than any other 

 Merostomata, having apparently just passed it to enter the 

 early subzoeal stage assumed by Limulus at a comparatively 

 early period in embryonic life. We appear to have, there- 

 fore, in Cyclus, whether it should prove to be a larval form, 

 or an adult, a long step backward towards the ancestral 

 nauplius form of the Merostomata. Cyclus, it should be 

 borne in mind, appeared during the Carboniferous period, 

 and affords some slight indication that the Merostomatous 

 type was developed at a considerably later period than the 

 Trilobites, as analogous forms among the Trilobites (Agnos- 

 tus) appear to me to conform much more to the type of the 

 larvffi of other Trilobites than does Cyclus to the larva of 

 Limulus. 



1 It will be seen by this arrangement that the Trilobites 

 are not regarded as in any way related to the Isopoda, as 

 has been suggested by the earlier writers, Professor Dana, and 

 more recently by Mr. Woodward, the general resemblance be- 

 ing one of analogy. Woodward remarks in the Geological 

 Magazine, July, 1871, p. 291, " In a note upon this specimen 

 (Billings' Asaphus), communicated to the Geological Society, 

 and published at the same time, I ventured to sugo-est that 

 the eviilence put forward by Mr. BiUings tended to place the 

 Trilobites near to, if not in, the Isopoda Normalia; and 

 that we might fairly expect to find that the Trilobita rep- 

 resented a more generalized type of structure than the 

 modern Isopods." Passing by certain direct homologies of the 

 adult Trilobites with the Phyllopoda and Merostomata, we 

 might add that the form of the larva is totally unlike that 

 of the young in the Isopoda. 



UEU01K3 BOST. SOC. NAT. HIST. VOL. II. 



