166 A. S. PACKARD, JR., ON THE 



Miiller (Fiir Darwin) in Ligia and other genera of Isopods, and by Prof. E. Van Beneden,^ 

 who has given the most careful description of it in Asellus, describing its origin. It is to 

 be borne in mind that the larval skin of the Isopods is a sac, as Miiller says "quite destitute 

 of appendages," which sarrounds the body of the embryo until it is ready to hatch. E. 

 Van Beneden remarks of this simple envelope in the Isopods, that "Fitz Miiller has demon- 

 strated the analogy which exists between the larval membrane of Ligia ^ and the nauplian 

 skin of the embryo of Mysis," ^ (a low decapod, " the near relationship of which with the 

 shrimps and prawns has recently again been generally recognized," as Miiller remarks). 

 " Both are the homologues of this membrane that the nauplii of the loAver Crustacea 

 secrete at their surface." Van Beneden then shows that of the two embryonal 

 membranes or skins, the first moult is a blastodermic moult, the second a nauplian, which 

 also corresponds to the larval membrane of Ligia, but differs in having appended to it the 

 rudiments of the first and second pairs of antenna}. 



This larval membrane of Limulus cannot be a nauplius skin,* because it is shed when the 

 embryo has attained to its true subzoeal state ; and the membrane instead of being a simple, 

 rounded sac as in Ligia, or with the appendages [i.e., the two pairs of antennae) as in Asel- 

 lus, or with these and the mandibles, as in Mysis, is provided with the full number of 

 appendages, and is thrown off like an ordinary moulted skin, and does not serve as a pro- 

 tective membrane. The embryo has simply outgrown its old skin and cast it off, just as it 

 moults from time to time after hatching. This embryonal moult is simply the first of a se- 

 ries of larval moults, and at the time of this first moult the larva has taken on its essen- 

 tial zoea-shape, with its abdomen differentiated from the head, and its two pairs of eyes, 

 simple and compound, and full complement of feet. 



Having at first, though with many misgivings, considered this moult a nauplian one, I am 

 led to consider it as rather analogous to the third moult described by E. Van Beneden, in 

 his beautiful memoir on Asellus. He shows that certain Anchorellne and some Lerneopods 

 after undergoing a blastodermic moult, accomplish a second (a nauplian moult), which is 

 formed "when the embryo is provided with three pairs of appendages characteristic of the 

 form of nauplius." This corresponds, he thinks, to the larval skin of Ligia and the nauplius 

 skin of Mysis. "In this second cuticle, the embryo continues to develop, and we see appear 

 a series of six pairs of appendages in the AnchorellEB, and five pairs in the Lerneopods. 

 The embryo attains the cyclopean form, which is, in our opinion, morphologically identical 

 with the form of zoea. It then secretes at its surfixce a third skin, which is the cyclopean or 

 zoean skin ; and the young Anchorella, as the young Lerneopod, comes into the world 



iRecberclies sur rEmbryogeiiie des Crustaces. — 1. Ob- ° " Tlie )-oiing animal inigbt be called a nauplius; but 



servations sur le Developpement de 1' Asellus aquaticus. essentially there is nothing but a rough copy of a nauplius 



Bulletins de I'Academie Royale des Sciences, etc., de Bel- skin, almost like a new egg-membrane, within which the 



gique. 2°' Ser. Tom. xxviii, 1869, p. 54. Mysis is developed." 1. c. p. 65. 



2 "As in Mysis also, a larval membrane is first of all ^ At the time an abstract of this paper was presented to 



formed, within which Ligia is developed. In Mysis this first the American Association for the Advancement of Science, 



larval skin maybe compared to a nauplius; in Ligia, it appears I called this a nauplius skin, but with much doubt at the 



like a maggot quite destitute of appendages, but produced into time. It is evident, however, that Limulus passes through a 



a long simple tail. The egg-membrane is retained longer nauplius state, at a time previous to possessing its full com- 



than in Mysis ; it bursts only when the limbs of the young plement of legs, during a period between that indicated by 



Lio-ia are already partially developed in their full number. figs. 15 and 16; though it does not shed a skin at this 



The dorsal surface of the Ligia is united to the larval skin a period, 

 little behind the head." FUr Darwin, English Trans, p. 69. 



