EMBRYOLOGY OF LIMULUS POLYPHEMUS. 217 



PI. XXXIII, figs. 1 and 2) Dr. Packard compares it with the free-swimming " nauplius " 

 condition of Entomostraca, especially of the Copepoda. He admits, however, that the 

 differences are very great, and that probably other observers will not share this opinion. 1 



The stage which follows (see PL XXXIII, figs. 3, 4, and 5, copied from Packard) is 

 distinguished by the differentiation of the thoracico-abdominal series, and the appearance 

 of a third pair of rudimentary thoracic limbs, together with the first indications of the 

 chela?. The compound eyes are also seen as two white dots on the third segment of the 

 cephalon. The ocelli appear still later and are situated near the centre of the anterior 

 border. The nine thoracico-abdominal rings are indicated, no more being added during 

 after life. There are six cephalic rings, indicated by deep emarginations on the head- 

 shield, reaching even to the glabella. The whole body in the embryo is spherical, being 

 as thick through as it is wide. " At this stage the chorion bursts open, the protoderm 

 increases greatly in size, aad a corresponding amount of sea water is absorbed (by 

 endosmosis) in which the embryo floats." 



Dr. Packard observed this larval skin moulted, the process occupying two or more 

 days owing to the want of aeration in the water, and the consequent sluggish condition 

 of the embryo. The skin was pulled off from before backwards, and might afterwards 

 be seen floating about in the protodermic fluid or entangled in the limbs of the embryo. 



He at first believed this to be a ' nauplius ' moult, but was subsequently led to con- 

 sider it as analogous to the third moult in Asellus, described by van Beneden 3 in his 

 Memoir on that genus. Prof, van Beneden has also pointed out that certain Anchorellse 

 and some Lerneopoda, after undergoing a first or blastodermic moult, accomplish a second 

 or nauplian moult, which occurs when the embryo is provided with three pairs of appen- 

 dages characteristic of the form of " Nauplius." This corresponds, he thinks, to the 

 larval skin of Ligia and the nauplius skin of My sis. 



" In this second cuticle the embryo continues to develop, and we see a series of six 

 pairs of appendages make their appearance in the Anchorellae and five in the Lernaeopoda. 

 It then secretes a third skin which is the cyclopean or zoeal skin, and the young Ancho- 

 rella and Lerneopod come into the world under this form (op. cit, p. 166). 



As the young Limulus increases in size the yolk is gradually absorbed, and the body 



1 We have elsewhere observed that " the greatest caution should be exercised in instituting com- 

 parisons between the so-called ' nauplius' and ' zoeal' stages of any one Crustacean when such stages are 

 passed within the egg and tbose of any other Crustacean whose young (as in the Copepoda) actually pass 

 through such stages after they have quitted the egg. 



" In the Decapoda we know of only one instance in which the young appears as a free-swimming 

 ' nauplius ;' in the majority we see only the ' zoeal ' and ' larval ' stages ; in some even the zoeal stage is 

 overleaped, and the young appears as a larva differing but little from the parent." — ['Encyclop. Brit.,' 

 9th edition, 1877, article "Crustacea," by H. Woodward, vol. vi, p. 650.] 



2 "Recherches sur l'Embryologenie des Crustaces. 1. Observations sur le Developpement de 

 V Asellus aquaticus" 'Bulletins de l'Academie Eoyale des Sciences, etc. Belgique,' 2me serie, torn, xxviii, 

 1869, p. 54. 



