ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 19.5 



stage, hence its connexion with Penseus has not been demonstrated at 

 either extremity of the chain of evidence. 



The little creature, according to Muller, is rather opaque and of a 

 brownish colour, darkest towards the extremities of the appendages. 



It is by these little appendages that the young animal swims, lashing 

 the water and working its way upwards to the light. 



The first change that is observable is that it becomes slightly larger, 

 and the terminal part projects into two pointed processes, terminating in 

 the two long caudal hairs which were previously present, and to which 

 others less important have been added. The number of hairs on the 

 natatory appendages have also increased. 



At this stage the form of the carapace is first indicated in the presence 

 of a transverse line. In this we perceive an important variation from 

 the forms of either the Cirripedia or decapod Crustacea, and moreover 

 contrary to that of the Euphausia as illustrated by Metschnikoff. 



In the youngest forms of Decapoda and Cirripeds the carapace is defined 

 from the earliest stages. 



In Lophogaster, according to Sars, the development resembles that of 

 Mysis. The form of the embryo is more annulose and the formation of 

 the great dorsal shield is more progressive. According to Fritz-Miiller 

 the development of the carapace in the young of Penaaus is upon the 

 same plan, and is first detected by the presence of a line immediately 

 behind the third pair of appendages. In the anterior pair may now be 

 seen that which after the next moult Fritz-Miiller takes to be the first 

 pair of antennae. The second pair becomes the second antennas, and the 

 third pair becomes the mandibles : close to which a large helmet-shaped 

 protuberance, which is taken to be the homologue of the anterior labrum, 

 is present. In this early stage Dr. Muller sees within the third pair of 

 appendages the mandibles with a prominent acute tooth and a broad 

 transversely furrowed masticatory surface, and he says that the mandible 

 must bear a non-setigerous appendage. Posterior to these three pairs of 

 lobes, the embryonic condition of the future oral appendages make their 

 appearance ; the eyes still continuing to be represented by a solitary 

 central organ. 



The rudimentary appendages exhibit within the sacs the presence of 

 hairs, which induced Dr. Muller to believe that after the next moult the 

 animal will pass into the Zocea stage. But here the progressive link is 

 broken in his researches, and there is nothing to demonstrate that this 

 Nauplius form passes into a Zooea stage more than the young of Mysis does. 

 Previously to the time that Muller found his Nauplius, Professor Sars 

 (1862)* studied the development of Lophogaster typicus, a Schizopod 

 belonging to the family Eupliausidm, and this he states to be precisely 

 similar to that of Mysis. 



In 1871 Metschnikoff communicated to ' Zeitschrif t fur Zool.' his ob- 

 servations on the young of Euphausia. The first specimens he found 

 in the open sea, and hypothetically assumed that they were the young of 

 Euphausia, although they were not in any way connected with the parent, 

 and had undergone one or two changes of form since quitting the ovum! 

 He says : "I was yet convinced that it by no means represented the 

 earliest larval form as it escaped from the ovum. I could only hypo- 



* Archiv. des Sci. Phys. et Nat., tome xxi., p. 87, and An. Nat. Hist., vol xii 18fi4 

 p. 461. ' "' » 



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