ARTHROPODA OF DOUBTFUL POSITION. 



85 



FIG. 117. Limulus embryo, nearly 

 ready to hatch. 



and the first two pair of abdominal limbs appear as flattened plates, resembling in 

 appearance the same appendages in the adult. Soon the cephalothorax and abdomen 

 become separated from each other, the limbs acquire 

 more of the adult character, and the abdomen becomes 

 segmented, while the jointing of the anterior part of 

 the body disappears. At about this time the egg ab- 

 sorbs a quantity of water, and by the pressure the egg- 

 shell is ruptured, but the embryo still remains within 

 the cellular membrane. It now shows the first move- 

 ments but in a very slight degree. The body is still 

 enclosed in the first larval skin, but within this the 

 spines of the sides of the abdomen and the hairs of the 

 gills are seen to develop gradually, pushing off the old 

 integument, although it does not really cast the skin 

 until it hatches or leaves the cellular membrane in 

 which it is enveloped. On leaving the egg the young 

 Limulus bears a very close resemblance to the young trilobite, as can be seen by a 

 comparison of figures 112 and 118. At this time the caudal spine has not been 



developed, existing only as an inconspicuous lobe at the 

 extremity of the abdomen. The sutures of the abdomen 

 have nearly disappeared. The young horseshoe swims 

 freely, and with the first moult the caudal spine begins 

 to assume the adult form. 



The question as to whether the horseshoe crab is a 

 crustacean or an arachnidan is too abstruse for consid- 

 eration here. The chief advocate of the latter view is 

 Professor E. Ray Lankester, and the arguments upon 

 which he places the most dependence are the nature 

 and homologies of the walking legs, and the origin of 

 the nerves which supply them ; the structure of the eye, 

 and a more or less perfect homology existing between 

 the gills of the horseshoe and the pulmonary lamellae of the scorpions, to which 

 we shall refer in a subsequent portion of this volume. Additional evidence for this 

 view is also found in the little which is known of the embryology. In the opinion 

 of the writer enough has been discovered to show that Limulus is not a crustacean, 

 but it has yet to be proved that it should be placed near the scorpions. The most 

 important objection to such a course are the gills, which are fitted for aquatic respira- 

 tion, and the absence of any trachea. 



The horseshoe crab frequents sandy and muddy shores, and especially sheltered 

 bays and estuaries where it is not exposed to the full force of the waves. It burrows 

 just beneath the surface, a life for which its structure is eminently adapted. The sharp 

 anterior edge of the thorax is peculiarly fitted for being forced through the sand, while 

 the caudal spine, and especially the sixth pair of feet with their whorls of flat spines, 

 afford a firm foothold for forcing the body forward. The crab first arches its body by 

 bending it at the joint between the cephalothorax and abdomen, draws the sixth pair 

 of legs forward, and then extends the body, pushing with all its might with the legs 

 and tail. Occasionally it leaves its underground burrows and moves over the bottom, 

 where its progression is a true walk, in which the last five pairs of legs are employed. 



FIG. 118. Limnlus, just hatched, 

 natural size and enlarged. 



