DEVELOPMENT OF LIMULUS POLYPHEMUS. 181 



In the freshly hatched zoea the body is divided into two regions : the cephalothorax (but 

 at first, as in Limulus, with no trace of thoracic segments or thoracic appendages, the two 

 pairs of large feet being deciduous maxillipedes, the thorax not being yet differentiated), 

 and a five to seven-jointed abdomen. The size of the cephalothorax, as compared with the 

 abdomen, varies greatly, as is well known, in the different forms of zoeoe, some forms resemb- 

 ling Eurypterus in the small cephalothorax. After the first moult five pairs of rudimentary- 

 thoracic limbs arise at the junction of the cephalothorax with the abdomen. We feel justi- 

 fied, then, in considering the larva of Limulus as a zoea. 



But while we have endeavored to show that the young of Limulus is essentially zoese- 

 form, we would not overlook the great diflFerences between the two forms, seen especially in 

 the elongated body, the form of the cephalothoracic limbs, and the presence and peculiar form 

 of the abdominal appendages of the decapodous zoea. The two pairs of antennaj (though 

 they are locomotive in function) of zoea, are in Limulus replaced by ambulatory feet, ending 

 in claws ; and the two pairs of biramose deciduous maxillipedes of the zoea are replaced by 

 feet identical in form with those in front. That there are a greater number of cephalothoracic 

 and abdominal appendages respectively in the zoea of Limulus than that of the Decapoda 

 is not of primary importance. When we examine the less essential characters of Limulus 

 we find, on comparison with certain forms of zoete, (such as Fritz Miiller's figure of the zoea 

 of a PenyDus, Fiir Darwin, fig. 32) with its large carapace and small abdomen, also Milne 

 Edward's figure of Erichthus aculeutus, with its large carapace and short, broad aljdomen, 

 with five pairs of swimming feet, many points of resemblance. The spines on the carapace 

 of Limulus are also comparable with those of certain zoea^. 



The Merostomata zoemform. As our Limulus at birth only essentially differs from the 

 adult by having three instead of six pairs of abdominal feet, and no caudal spine (though 

 existing in a rudimentary state), the question resolves itself into a comparison of the adult 

 Limulus with the young of the Decapoda. In some respects there is a certain likeness be- 

 tween the higher Branchiopoda (the Phyllopoda especially) and the zoea of a Decapod. 

 This consists in the fact that all the limbs between the mouth and the abdomen, or genital 

 openings, are but slightly differentiated — in short, they are all maxillipedes, and there is an 

 absence of the five pairs of special locomotive limbs, to which the 3'onng Decapoda owe their 

 name, appended to a thoracic region (pereion), which distinguishes the zoea, as Miiller well 

 says. In Branchipus, Apus and Nebalia, with their two pairs of anteimse, we have the 

 nearest approach to a zoea. The Phyllopoda have hi- or tri-ramose abdominal appendages, 

 and while no zoea has been hitherto known to possess them, the zoea of Alplieus minus Say 

 has five well developed pairs. The telson, as in the Merostomata and certain Phyllopoda 

 (Apus and Limnadia) "consists of a single piece." Prof. Huxley^ has compared Pterygotus 

 (Himantopterus) to the Stomapoda and zoea forms. 



While the likeness of Limulus to a zoea may seem very remote, we would call attention 

 to certain peculiar decapodous larvae, discovered and figured by Dr. A. Dohrn, which bear 

 a certain resemblance to the king crab. Such are the " zoea of a Pen8eus(?)" represented 

 at fig. 54, pi. xxx, of Siebold and Kolliker's Zeitschriffc fiir Wissens. Zoologie " (Bd. 21, Heft. 



^ "The nearest approach to Himantopterus which could bu the appendages as we find among the zoefeform Macruran 



constructed out of the elements afforded by existing Cms- larva;." Observations on the Structure and Affinities of Hi- 



tacea, then, would be produced by superinducing upon the mantopterus, Quarterly Journal Geological Society of Lon- 



general form of a Cumoid crustacean such a modification of don, xii., 1856, p. 37. 



MEMOIRS BOST. SOC. NAT. HIST. VOX.. II. 46 



