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Another intricate matter is the development of Penæus. Fritz Müller in 1864 believed himself to have discovered the earliest stage. Of the brood of some prawns belonging to Penæus or some immediately kindred genus, he says, 'they quit the egg with unsegmented oval body, an unpaired frontal eye, and three pairs of swimming-feet, of which the first are simple, the other two two-branched, belonging, therefore, to the larval form so frequent among the lower Crustacea, to which O. F. Müller gave the name of Nauplius. No indication of a carapace, of the paired eyes, of mouth-organs near the mouth which is over-arched by a helmet-shaped hood!' Between this and the adult there are various Zoea and Mysis or Schizopod stages, not to mention the Protozoea of Claus interposed between the egg and the Nauplius form. Spence Bate alludes to the claim made by Professor Brooks in 1882 that, having captured and kept in confinement a specimen, he had witnessed every moult between the youngest Protozoea and the young Penaeus, but against this is set the comment of Mr. Walter Faxon in 1883 that Professor Brooks' 'youngest Protozoea is an older stage than the youngest stage secured by Fritz Müller,' to which he adds that no observer has rediscovered Müller's Nauplius.' Hence Spence Bate himself says that 'two links of importance are yet wanting: the one is that which connects the earliest Protozoea form with Fritz Müller's Nauplius, and the other that which connects the Nauplius with Penæus; either of these being demonstrated will prove the connection, and establish the splendid hypothesis of Fritz Müller.'

Solenocěra, Lucas, 1850, with its Mediterranean species Solenocera Philippii, Lucas, is by Victor Carus made a synonym of Penaeus siphonoceras, Philippi, but it differs from Penæus in having the flagella of the first antennæ longer than the carapace, and should therefore be called Solenocera siphonoceras (Philippi), the earlier name Penæus membranaceus, Milne-Edwards, having been already used by Risso for a different species. The flagella in question are rather remarkable, since the primary is very slender,

whereas the secondary is dilated and longitudinally hollowed so that its companion can be sheltered within it when not in use, but at other times the two pairs of flagella together form the efferent branchial tube, which is continued backwards by the peduncles of the first and the scales of the second antennæ, these making a broad channel between the bases of the peduncles of the second antennæ, where it is closed in below by the mandibular 'palp,' and diverges on each side of the upper lip into the passages from the branchial chambers. The generic and specific names alike signify a creature with channel- or pipe-forming antennæ.'

Pleoticus, Spence Bate, 1888, also has the flagella of the first antennæ longer than the carapace, but without the grooved arrangement. Its second antennæ claim notice as having the flagellum'three times the length of the animal, or more.'

Sicyonia, Milne-Edwards, 1830, has its species, two of which occur in the Mediterranean, distinguished for the rigidity of the integument. The flagella of the first antennæ are very short; there are no exopods to the trunk-legs as there are in Penceus, and the pleopods are all single-branched. From Penaeus it differs in the structure and arrangement of the branchiæ, though agreeing with it apparently in the absence of podobranchiæ. In defining the genus Spence Bate says that the second maxillipeds carry a mastigobranchial plate without a podobranchia,'' one arthrobranchial and one pleurobranchial plume.' On the next page, after giving a scheme of the branchiæ of Sicyonia which includes six pleurobranchiæ and no podobranchiæ, he states that it differs from Penæus in the absence of any traces of pleurobranchiæ, in the reduction of the arthrobranchial plumes, and in the presence of one podobranchial plume attached to the first pair of gnathopoda' [i.e. second maxillipeds]. Presently after, in the description of Sicyonia carinata (Olivier), he says of these same second maxillipeds that the first joint 'carries a long and slender mastigobranchia shaped like that in Pencus, and, as in that genus, there is no branchial plume attached to it.' Thus there both is and is not

THOUSANDS OF FATHOMS

219 a podobranchia to the second maxillipeds, and there are no traces of pleurobranchiæ although pleurobranchiæ are developed on six pairs of appendages. These are riddles which those who have specimens to compare with the descriptions may be able to solve.

Aristeus, Duvernoy, 1841, is distinguished from Penæus chiefly by the circumstance that on the second and third maxillipeds and the first three trunk-legs it has the podobranchia which the other is without. Aristeus antennatus (Risso) occurs in the Mediterranean and Atlantic, and is described as having a smooth pleon.

Hepomadus, Spence Bate, 1881, is distinguished from Aristeus by a hepatic tooth on the shoulder of the carapace. Hepomadus glacialis, of which Spence Bate's figures are given in the adjoining Plate on a reduced scale, was taken near Yokohama at the frigid depth of 1,875 fathoms.

Several new genera from Atlantic exploring expeditions have been described in recent years by Professor S. I. Smith, as Hymenopenæus, 1882, meaning the membranaceous Penæus, Amalopenæus, 1882, which, at least in the type species Amalopenæus elegans, has only the sixth segment of the pleon carinate. Some of the lately described genera have names alluding to the great depths from which they were obtained Benthesicymus, Spence Bate, 1881, Benthocetes, S. I. Smith, 1884, Benthonectes, S. I. Smith, 1885, all meaning those that dwell or swim in the abysses of the billowy ocean. Benthesicymus has a submembranous integument, exopods to the limbs as in Penæus, podobranchiæ as in Aristeus, and the last two pairs of trunklegs longer than the preceding pairs. One of the species, Benthesicymus pleocanthus, Spence Bate, was trawled from a depth of 3,050 fathoms in the North Pacific, while other specimens were taken twenty degrees further south in the smaller depths of 450 and 1,050 fathoms. Benthonectes is specially characterised by the multiarticulate flagelliform dactyli, that is the subdivided terminal joints, of the last two pairs of trunk-legs. Xiphopeneus, S. I. Smith, 1885, and Benthocetes have a corresponding peculiarity in the propodi or sixth joint of the same limbs. In some of the