174 



A. S. PACKARD, JR., ON THE 



Fig. 28. — Nauplius of Penjeus. 



its full number of legs, we should not be justified in calling it a nauplius, or, on the other 

 hand, a zoea, in the full sense of the word, and for its singular form I would use the pro- 

 visional term suhzoea. The larval form is, like 

 that of the Trilobites, a remarkably exceptional 

 one, and corresponds with its mode of life ; for, as 

 we have seen above, it immediately after hatching 

 creeps over the surface of the sand or mud, or 

 burrows deep after its food. Swimming is an 

 exceptional act, as I have found by watching 

 them day after day for three months in succession. 

 Hence their mode of life does not demand a nau- 

 plian, or complete zoea form. 



Among other exceptional features in the egg is 

 the remarkably tough chorion, and the protoderm, 

 with its cells retained, and even enlarged, which, 

 so far as I am aware, is an exception to the eggs 

 of other Crustacea, and of insects; as also its 

 enormous increase in size during the growth of the embryo. It is well known that the eggs 

 of many insects and Crustacea do increase, and even double in size ; but this is generally 

 attended by a change in form of the embryo, which from being round becomes ovate ; no in- 

 stance, however, is known to me among the Arthropoda where the amnion increases four or 

 five times in size, as that of Limulus does, and where, at the same time, the membrane is 

 perfectly spherical. 



Homologies of the Merostomata. Regarding the number of segments in the body of 

 Pterygotus, Mr. H. Woodward^ remarks : " In describing the divisions of the body in Pterygo- 

 tus Anglicus, we have spoken of the head and the body, avoiding the terms ' thoracic ' and 

 ' abdominal somites ' as much as possible. On reference to the restored figure at plate viii, 

 fig. 1, it will be seen that there is evidence, according to the paired appendages, of seven 

 somites having coalesced to form the cephalic division. Assuming that all the segments 

 united to form the head are represented by pairs of appendages, we have then a true ceph- 

 alon ; the seven succeeding free somites are thoracic, the abdomen being represented by 

 five somites and a telson, or terminal joint. But assuming the antennules to be wanting 

 [according to Huxley's opinion], we must conclude the head to represent eight coalesced 

 segments, that is to say, seven cephalic and one thoracic, the latter bearing the ' opercu- 

 lum,' or thoracic plate. In this case only the first six anterior somites will be counted as 

 thoracic, the posterior six will be reckoned as abdominal, and the ' telson ' will make up the 

 twenty-one segments. The likeness which Pterygotus offers to Limulus is very much 

 strengthened by this latter view. As we proceed to the illustration of the other genera 

 belonging to this order, we shall be still more strongly impressed with the many points of 

 close resemblance which they will be found to exhibit." 



Having observed that the ocelli of Limulus are developed on the first segment of the 

 head, which is to be regarded as the first antennary segment, and that the compound eyes 

 are developed on the tergal side of the third, or mandibulary segment, I have been led to 



1 Monograph of the Fossil Merostomata of Great Britain. Memoirs of the Palaeontographical Society. London, 1866. 



