Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/686

Rh 652 CRUSTACEA pair of minute antennae, two anterior horns, which inclose tlie second pair of antennae, 1 one pair of uniramous and two pairs of biramous natatory legs, a forked terminal projection t &amp;gt; the b-&amp;gt;dy, and a posterior point to t 3 carapace (see fig. 60 A, and fig. 61). The nauplii of the Anchoracepha- 1 1 are distinguished from the Copepoda by possessing a dorsal shield or ca- rapica, which some times, as in Saccu- lini purpurea, pro jects far beyond the body all round. They a re also farther distinguish ed by possessing a pair of SO-Called FIG. 61. Nauplius of TetracUta porosa after the first 11 nlfi ^rorv fi 1 o- moult. Magn. 90 diam. The brain is seen surround- ~ i ! ng the eye and fl &quot; om ifc the Ifactol y filaments ments. Which issue. Behind it are some delicate muscles passing spring iirectly from the buccal hood (Fritz * m{ *-&amp;gt; the head. These filaments, or horns of the carapace (which are interpreted as the second pair of antennas by Darwin), are believed by Fritz Miiller to be the homologue of the so-called &quot;green gland,&quot; which opens at the end of a conical process at the base of the inferior antennas in the Decapoda, and of the conical process, with an efferent duct traversing it, seen on the inferior antennae of the Amphi- poda. Tue abdomen of the young Cirripede is produced into a long tail-like furcate extremity, that of the young in Rlrizocepliala into a movable caudal fork. The young Cirripedes have a mouth, stomach, &c., and their posterior pairs of limbs are fitted for organs of prehension and miuducation. In the young of llhizocephala all these organs are wanting. The youug Cirripede, having to undergo several moults as a nauplius, is provided with organs to sustain its life. The young Rhizocephalon being astomatois, cannot sustain life in its nauplius state for long, and must therefore more rapidly pass through its transform xtious. They b )th at length arrive at an equally ast&amp;gt;mitous p.-^.i-stage. In this stage we see the young animil with its carapac3 folded together like a bivalved shall ; tin foremost limbs become transformed into very peculiar adherent feet, and the two fol lowing pairs, like the frontal horns, are cist off with tlie naupliu 5-skin. Behinl those are ,, .. r . e . Fir, 2. Pupa of a Balanide (CJitha- SIX pairs Of powerful biramOSO maltat) Ma^.i. CO diam. The ririrorv fpor wirli IrmT pf-r adherent feet are retracted within u uatory i ,et, wicn long setae the rathsr op!1 q lie ttnt erior part of and two short setigerous caudal thesheii. (fritz nuiier) appendages. The young pupre of the Rhizocephala and Cirripedia agree in every particular, save that the latter possesses a pair of composite eyes ; sometimes also traces of the frontal horns seem to persist. When the proper time arrives the pupae of the Cirripedia nttach. themselves by means of their prehensile antennae to r )cks, shells, turtles, cetacea, drift-wood, ships, &c. ; the carapace becomes converted into the peculiar sessile-shell of the Balanus, or the pedunculated Lep-is ; the natatory feet grow into long cirri by which nourishment is whirled to the mouth, now open and furnished with mandibles and ff 63. Pupaof Saccuhna purpurea. Matrn. 180 diam. The filaments on the adherent feet may be the com- mencements of the future roots. (Fritz Muller.) 1 &quot;Olfactory filaments&quot; (Fritz Muller). maxilla?. The pupaa of Rhizocephala in like manner attach themselves to the abdomen of crabs, Porcellance, and hermit- crabs ; but they remain astomatous, lose all their limbs completely, and appear as sausage-like, sack-shaped, or discoidal excrescences upon their host, filled with ova (figs. 82 and 83) ; from the point of attachment closed tubes, ramifying like roots, sink deep into the inte rior of the host, twisting around the intestine, or are diffused among the Sac-like tubes of the liver The .. only manifesta tions of life which persist in this most retrogressively meta morphosed Crustacean are powerful contractions of the roots and alternate expansion and contraction of the body, causing water to flow into the brood -cavity, to Le again expelled through a wide orifice (Fritz Muller). Darwin lias recorded various anomalous Cases of development in the Cirripedia ; amongst others, that of Cryptophialus minutus (which forms a separate section Abdominalia, Darwin), parasitic in the shell of Conchotepas peruviana. The egg, at first elliptical, becomes broader anteriorly, then acquires three club-shaped horns, one at each anterior angle and one behind. Subsequently the posterior horn disappears, ani the adherent feet may be re cognized within the anterior ones. From this &quot; egg-like larva &quot; the pupa is directly produced. Its carapace is but slightly compressed laterally, and is hairy as in Saccuima purpurea ; the adherent feet are large, the natatory feet wanting, as are also the corresponding cirri in the adult animal. Mr Spence Bate mentions a similar case in a Rhizocephalon, in which the nauplius-stage is overleaped and the young quits the eggs as a pupa-form larva. EXUVIATION IN THE ADULT AND REPAKATION OP INJURIES. As we have already seen, the young Crustacean, on quitting the egg, usually undergoes a series of larval metamorphoses more or less numerous, and subject to con siderable variation even among closely-allied forms. Eventually, whether by a direct or an indirect route, a form is attained in all, which, save in size, closely agrees with the adult. Amongst the Insecta the larva usually undergoes repeated moults during its growth, from the time when it first quits the egg until it reaches the pupa-stage, a period of rest in most, but not in all insects, 2 and an astomatous stage in some larval Crustacea. 3 From the pupa springs the full- grown and perfect insect, when no further moult or change takes place indeed, in some insects the parent only lives to deposit its eggs, and then dies. The immature Crusta cean, in passing through its nauplial and zoeal stages, may moult its skin seven or eight times, or even more ; never theless, when it reaches the imago stage, 4 it has not nearly attained the size of the adult parent, but continues to grow and cast its calcareous envelope as often ^as it? increased size necessitates its so doing. When adult, it The aquatic pupa of the dragon-fly is active and predaceous. 3 The pupte of Rhizocephala and Cirripedia are both astomatous. swimming astomatous larva in the Cirripedia previous to its settling down, casting off its pupa-coat, and becoming adult. The writer has long doubted if the Crustacea ever really arrive at this highest or imago condition, and whether they are not always in a &quot; pupa-period all their lives. The larval Aphis the branchiated Axolotl are but arre stages of development of more advanced forms, but they deposit egg?, and in the case of the Axolotl they possess all the attributes of Ufl perfect animal, save the persistent external larval branchire.
 * Imago. Darwin has applied the term &quot;pupa-stage to the free-