Schulers Books Online
books - games - software - wallpaper - everything
- Facts and Arguments for Darwin - 10/20 -
present in the young animals, disappears without leaving a trace in the adults.
The young animal, whilst still in the egg, acquires the full number of its segments and limbs. In cases where segments are amalgamated together, such as the last two segments of the thorax in Dulichia, the last abdominal segments and the tail in Gammarus ambulans and Corophium dentatum, n. sp., and the last abdominal segments and the tail in Brachyscelus,* or where one or more segments are deficient, as in Dulichia and the Caprellae, we find the same fusion and the same deficiencies in young animals taken out of the brood-pouch of their mother. (* According to Spence Bate, in Brachyscelus crusculum the fifth abdominal segment is not amalgamated with the sixth (the tail) but with the fourth, which I should be inclined to doubt, considering the close agreement which this species otherwise shows with the two species that I have investigated.) Even peculiarities in the structure of the limbs, so far as they are common to both sexes, are usually well-marked in the newly hatched young, so that the latter generally differ from their parents only by their stouter form, the smaller number of the antennal joints and olfactory filaments, and also of the setae and teeth with which the body or feet are armed, and perhaps by the comparatively larger size of the secondary flagellum. An exception to this rule is presented by the Hyperinae which usually live upon Acalephae. In these the young and adults often have a remarkably different appearance; but even in these there is no new formation of body-segments and limbs, but only a gradual transformation of these parts.*
(* In the young of Hyperia galba Spence Bate did not find any of the abdominal feet, or the last two pairs of thoracic feet, but this very remarkable statement required confirmation the more because he examined these minute animals only in the dried state. Subsequently I had the wished-for opportunity of tracing the development of a Hyperia which is not uncommon upon Ctenophora, especially Beroe gilva, Eschsch. The youngest larva from the brood-pouch of the mother already possess THE WHOLE of the thoracic feet; on the other hand, like Spence Bate, I cannot find those of the abdomen. At first simple enough, all these feet soon become converted, like the anterior feet, into richly denticulated prehensile feet, and indeed of three different forms, the anterior feet (Figure 44) the two following pairs (Figure 45) and finally the three last pairs (Figure 46) being similarly constructed and different from the rest. In this form the feet remain for a very long time, whilst the abdominal appendages grow into powerful natatory organs, and the eyes, which at first seemed to me to be wanting, into large hemispheres. In the transition to the form of the adult animal the last three pairs of feet (Figure 49) especially undergo a considerable change. The difference between the two sexes is considerable; the females are distinguished by a very broad thorax, and the males (Lestrigonus) by very long antennae, of which the anterior bear an unusual abundance of olfactory filaments.
Their youngest larvae of course cannot swim; they are helpless little animals which firmly cling especially to the swimming laminae of their host; the adult Hyperiae, which are not unfrequently met with free in the sea, are, as is well known, the most admirable swimmers in their order. ("Il nage avec une rapidite extreme," says Van Beneden of H. Latreillii M.-Edw.)
The transformation of the Hyperiae is evidently to be regarded as ACQUIRED and not INHERITED, that is to say the late appearance of the abdominal appendages and the peculiar structure of the feet in the young are not to be brought into unison with the historical development of the Amphipoda, but to be placed to the account of the parasitic mode of life of the young.
As in Brachyscelus, free locomotion has been continued to the adult and not to the young, contrary to the usual method among parasites. Still more remarkable is a similar circumstance in Caligus, among the parasitic Copepoda. The young animal, described by Burmeister as a peculiar genus, Chalimus, lies at anchor upon a fish by means of a cable springing from its forehead, and having its extremity firmly seated in the skin of the fish. When sexual maturity is attained, the cable is cut, and the adult Caligi, which are admirable swimmers, are not unfrequently captured swimming freely in the sea. (See 'Archiv. fur Naturgeschichte' 1852 1 page 91).)
(FIGURES 44 TO 46. Feet of a half-grown Hyperia Martinezii, n. sp. (Named after my valued friend the amiable Spanish zoologist, M. Francisco de Paula Martinez y Saes, at present on a voyage round the world.)
FIGURES 47 TO 49. Feet of a nearly adult male of the same species; 44 and 47 from the first pair of anterior feet (gnathopoda); 44 and 48 from the first, and 46 and 49 from the last pair of thoracic feet. Magnified 90 diam.)
Thus, in order to give a few examples, the powerful chelae of the antepenultimate pair of feet, of Phromina sedentaria, are produced, according to Pagenstecher, from simple feet of ordinary structure; and vice versa, the chelae on the penultimate pair of feet of the young Brachyscelus, become converted into simple feet. In the young of the last-mentioned genus the long head is drawn out into a conical point and bears remarkably small eyes; in course of growth, the latter, as in most of the Hyperinae, attain an enormous size, and almost entirely occupy the head, which then appears spherical, etc.
The difference of the sexes which, in the Gammarinae is usually expressed chiefly in the structure of the anterior feet (gnathopoda, Sp. Bate) and in the Hyperinae in the structure of the antennae, is often so great that males and females have been described as distinct species, and even repeatedly placed in different genera (Orchestia and Talitrus, Cerapus and Dercothoe, Lestrigonus and Hyperia) or even families (Hyperines anormales and Hyperines ordinaires). Nevertheless it is only developed when the animals are nearly full-grown. Up to this period the young resemble the females in a general way, even in some cases in which these differ more widely than the males from the "Type" of the order. Thus in the male Shore-hoppers (Orchestia) the second pair of the anterior feet is provided with a powerful hand, as in the majority of the Amphipoda, but very differently constructed in the females. The young, nevertheless, resemble the female. Thus also,--and this is an extremely rare case,* (* "I know of no case in which the inferior (antennae) are obsolete, when the superior are developed," Dana. (Darwin, 'Monograph on the Subclass Cirripedia, Lepadidae' page 15.)--the females of Brachyscelus are destitute of the posterior (or inferior) antennae; the male possesses them like other Amphipodae; in the young I, like Spence Bate, can find no trace of them.
It is, however, to be particularly remarked, that the development of the sexual peculiarities does not stand still on the attainment of sexual maturity.
(FIGURE 50. Foot of the second pair ("second pair of gnathopoda") of the male of Orchestia Tucurauna, magnified 15 diam.
FIGURE 51. Foot of the second pair ("second pair of gnathopoda") of the female of Orchestia Tucurauna, magnified 15 diam.)
For example, the younger sexually mature males of Orchestia Tucurauna, n. sp., have slender inferior antennae, with the joints of the flagellum not fused together, the clasping margin ("palm," Sp. Bate) of the hand in the second pair of feet is uniformly convex, the last pair of feet is slender and similar to the preceding. Subsequently the antennae become thickened, two, three, or four of the first joints of the flagellum are fused together, the palm of the hand acquires a deep emargination near its inferior angle, and the intermediate joints of the last pair of feet become swelled into a considerable incrassation. No museum-zoologist would hesitate about fabricating two distinct species, if the oldest and youngest sexually mature males were sent to him without the uniting intermediate forms. In the younger males of Orchestia Tucuratinga, although the microscopic examination of their testes showed that they were already sexually mature, the emargination of the clasping margin of the hand (represented in Figure 50) and the corresponding process of the finger, are still entirely wanting. The same may be observed in Cerapus and Caprella, and probably in all cases where hereditary sexual differences occur.
(FIGURE 52. Male of a Bodotria, magnified 10 diam. Note the long inferior antennae, which are closely applied to the body, and of which the apex is visible beneath the caudal appendages.)
Next to the extensive sections of the Stalk-eyed and Sessile-eyed Crustacea, but more nearly allied to the former than to the latter, comes the remarkable family of the Diastylidae or Cumacea. The young, which Kroyer took out of the brood-pouch of the female, and which attained one-fourth of the length of their mother, resembled the adult animals almost in all parts. Whether, as in Mysis and Ligia, a transformation occurs within the brood-pouch, which is constructed in the same way as in Mysis, is not known.* (* A trustworthy English Naturalist, Goodsir, described the brood-pouch and eggs of Cuma as early as 1843. Kroyer, whose painstaking care and conscientiousness is recognised with wonder by every one who has met him on a common field of work, confirmed Goodsir's statements in 1846, and, as above mentioned, took out of the brood-pouch embryos advanced in development and resembling their parents. By this the question whether the Diastylidae are full-grown animals or larvae, is completely and for ever set at rest, and only the famous names of Agassiz, Dana and Milne-Edwards, who would recently reduce them again to larvae (see Van Beneden, 'Rech. sur la Fauna littor. de Belgique' Crustacees pages 73 and 74), induce me, on the basis of numerous investigations of my own, to declare in Van Beneden's words; "Parmi toutes les formes embryonnaires de podophthalmes ou d'edriophthalmes que nous avons observees sur nos cotes, nous n'en avons pas vu une seule qui eut meme la moindre resemblance avec un Cuma quelconque." The ONLY THING that suits the larvae of Hippolyte, Palaemon and Alpheus, in the family character of the Cumacea as given by Kroyer which occupies three pages (Kroyer, 'Naturh. Tidsskrift, Ny Raekke,' Bd. 2 pages 203 to 206) is: "Duo antennarum paria." And this, as is well known, applies to nearly all Crustacea. How well warranted are we therefore in identifying the latter with the former. However, it is sufficient for any one to glance at the larva of Palaemon (Figure 27) and the Cumacean (Figure 52) in order to be convinced of their extraordinary similarity!) The caudal portion of the embryo in the Diastylidae, as I have recently observed, is curved upwards as in the Isopoda, and the last pair of feet of the thorax is wanting.
Equally scanty is our knowledge of the developmental history of the Ostracoda. We know scarcely anything except that their anterior limbs are developed before the posterior one (Zenker). The development of Cypris has recently been observed by Claus:--"The youngest stages are shell-bearing Nauplius-forms."
CHAPTER 9. DEVELOPMENTAL HISTORY OF ENTOMOSTRACA, CIRRIPEDES, AND RHIZOCEPHALA.
The section of the Branchiopoda includes two groups differing even in their development,--the Phyllopoda and the Cladocera. The latter minute animals, provided with six pairs of foliaceous feet, which chiefly belong to the fresh waters, and are diffused under similar forms over the whole world, quit the egg with their full number of limbs. The Phyllopoda, on the contrary, in which the number of feet varies between 10 and 60 pairs, and some of which certainly live in the saturated lie
Previous Page Next Page
1 5 6 7 8 9 10 11 12 13 14 15 20
Schulers Books Online
books - games - software - wallpaper - everything