Facts and Arguments for Darwin, Fritz Muller [ebook reader computer .TXT] 📗

The youngest Nauplius (Figure 28) is immediately followed by forms in which a fold of skin runs across the back behind the third pair of feet, and four pairs of stout processes (rudiments of new limbs) sprout forth on the ventral surface. Within the third pair of feet, powerful mandibles are developed.

In a subsequent moult the new limbs (maxillae, and anterior and intermediate maxillipedes) come into action, and in this way the Nauplius becomes a Zoea (Figure 29), agreeing perfectly with the Zoea of the Crabs in the number of the appendages of the body, although very different in form and mode of locomotion and even in many particulars of internal structure. The chief organs of motion are still the two anterior pairs of feet, which are slender and furnished with long setae; the third pair of feet loses its branches, and becomes converted into mandibles destitute of palpi. The labrum acquires a spine directed forward and of considerable size, which occurs in all the Zoeae of allied species. The biramose maxillipedes appear to assist but slightly in locomotion. The forked tail reminds us rather of the forms occurring in the lower Crustacea, especially the Copepoda, than of the spatuliform caudal plate which characterises the Zoeae of Alpheus, Palaemon, Hippolyte, and other Prawns, of the Hermit Crabs, the Tatuira and the Porcellanae. The heart possesses only one pair of fissures, and has no muscles traversing its interior like trabeculae, whilst in other Zoeae two pairs of fissures and an interior apparatus of trabeculae are always distinctly recognisable.

During this Zoeal period the paired eyes, the segments of the middle-body and abdomen, the posterior maxillipedes, the lateral caudal appendages and the stump-like rudiments of the feet of the middle-body are formed (Figure 30). The caudal appendages sprout forth like other limbs freely on the ventral surface, whilst in other Prawns, the Porcellanae, etc., they are produced in the interior of the spatuliform caudal plate.

As the feet of the middle-body come into action, simultaneously with other profound changes, the Zoea passes into the Mysis-or Schizopod-form (Figure 31). The antennae cease to serve for locomotion, their place is taken by the thoracic feet, furnished with long setae, and by the long abdomen which just before was laboriously dragged along as a useless burden, but now, with its powerful muscles, jerks the animal through the water in a series of lively jumps. The anterior antennae have lost their long setae, and by the side of the last (fourth) joint, endowed with olfactory filaments, there appears a second branch, which is at first of a single joint. The previously multi-articulate outer branch of the posterior antennae has become a simple lamella, the antennal scale of the Prawn; beside this appears the stump-like rudiment of the flagellum, probably as a new formation, the inner branch disappearing entirely. The five new pairs of feet are biramose, the inner branch short and simple, the outer one longer, annulated at the end, furnished with long setae, and kept, as in Mysis, in constant whirling motion. The heart acquires new fissures, and interior muscular trabeculae.

During the Mysis-period, the auditory organs in the basal joint of the anterior antennae are formed; the inner branches of the first three pairs of feet are developed into chelae and the two hinder pairs into ambulatory feet; palpi sprout from the mandibles, branchiae on the thorax, and natatory feet on the abdomen. The spine on the labrum becomes reduced in size. In this way the animal gradually approaches the Prawn-form, in which the median eye has become indistinct, the spine of the labrum, and the outer branches of the cheliferous and ambulatory feet have been lost, the mandibular palpi and the abdominal feet have acquired distinct joints and setae, and the branchiae come into action.

In another Prawn, the various larval states of which may be easily recognised as belonging to the same series by the presence of a dark-yellow, sharply-defined spot surrounding the median eye, the youngest Zoea (Figure 32), probably produced from the Nauplius, agrees in all essential particulars with the species just described; its further development is, however, very different, especially in that neither the feet of the middle, nor those of the hind-body are formed simultaneously, and that a stage of development comparable to Mysis in the number and structure of the limbs does not occur.

(FIGURE 32. Youngest (observed) Zoea of another Prawn. The minute buds of the third pair of maxillipedes are visible. The formation of the abdominal segments has commenced. Paired eyes still wanting. Magnified 45 diam.)

Traces of the outer maxillipedes make their appearance betimes. Then feet appear upon four segments of the middle-body, and these are biramose on the three anterior segments, and simple, the inner branch being deficient, on the fourth segment. On the inner branches the chelae are developed; the outer branches are lost before an inner branch has made its appearance on the fourth segment (Figure 32). The latter again becomes destitute of appendages, so that in this case at an early period four, and at a later only three, segments of the middle-body bear limbs. The fifth segment is still entirely wanting, whilst all the abdominal segments have also acquired limbs, and this one after the other, from before backwards. The adult animal, as shown by the three pairs of chelae, will certainly be very nearly allied to the preceding species. ( The oldest observed larvae (see Figure 33) are characterised by the extraordinary length of the flagella of the outer antennae, and in this respect resemble the larva of Sergestes found by Claus near Messina (Zeitschr. fur Wiss. Zool. Bd. 13 Taf 27 Figure 14). This unusual length of the antennae leads to the supposition that they belong to our commonest Prawn, which is very frequently eaten, and is most nearly allied to Peneus setiferus of Florida. Claus’s Acanthosoma (l.c. Figure 13) is like the younger Mysis-form of the larva figured by me in the ‘Archiv fur Naturgeschichte,’ 1836, Taf 2, Figure 18, and which I am inclined to refer to Sicyonia carinata.)

The youngest larva of the Schizopod genus Euphausia observed by Claus, stands very near the youngest Zoea of our Prawns; but whilst its anterior antennae are already biramose, and it therefore appears to be more advanced, it still wants the middle maxillipedes. In it also Claus found the heart furnished with only a single pair of fissures. Do not Nauplius-like states in this case also precede the Zoea?

(FIGURE 33. Older larva produced from the Zoea represented in Figure 32. The last segment and the last two pairs of feet of the middle-body are wanting. Magnified 20 diam.)

The developmental history of Mysis, the near relationship of which with the Shrimps and Prawns has recently again been generally recognised, has been described in detail by Van Beneden. So far as I have tested them I can only confirm his statements. The development of the embryo commences with the formation of the tail! This makes its appearance as a simple lobe, the dorsal surface of which is turned towards and closely applied to that of the embryo. (The young of other Stalk-eyed Crustacea are, as is well known, bent in the egg in such a manner that the ventral surfaces of the anterior and posterior halves of the body are turned towards each other,—in these, therefore, the dorsal, and in Mysis the ventral surface appears convex.) The tail soon acquires the furcate form with which we made acquaintance in the last Prawn-Zoea described. Then two pairs of thick ensiform appendages make their appearance at the opposite end of the body, and behind these a pair of tubercles which are easily overlooked. These are the antennae and mandibles. The egg-membrane now bursts, before any internal organ, or even any tissue, except the cells of the cutaneous layer, is formed. The young animal might be called a Nauplius; but essentially there is nothing but a rough copy of a Nauplius-skin, almost like a new egg-membrane, within which the Mysis is developed. The ten pairs of appendages of the fore- (maxillae, maxillipedes) and middle-body make their appearance simultaneously, as do the five pairs of abdominal feet at a later period. Soon after the young Mysis casts the Nauplius-envelope it quits the brood-pouch of the mother. ( Van Beneden, who regards the eye-peduncles as limbs, cannot however avoid remarking upon Mysis: “Ce pedicule n’apparait aucunement comme les autres appendices, et parait avoir une autre valeur morphologique.”)

For some time, owing to an undue importance being ascribed to the want of a particular branchial cavity, Mysis, Leucifer, and Phyllosoma were referred to the Stomapoda, which are now again limited, as originally by Latreille, to the Mantis-shrimps (Squilla), the Glass-shrimps (Erichthus) and their nearest allies. Of the developmental history of these we have hitherto been acquainted with only isolated fragments. The tracing of the development in the egg is rendered difficult by the circumstance, that the Mantis-shrimps do not, like the Decapoda, carry their spawn about with them, but deposit it in the subterranean passages inhabited by them in the form of thin, round, yellow plates. The spawn is consequently exceedingly difficult to procure, and unfortunately it becomes spoilt in a day when it is removed from its natural hatching place, whilst on the contrary the progress of development may be followed for weeks together in the eggs of a single Crab kept in confinement. The eggs of Squilla, like those removed from the body of the Crab, die because they are deprived of the rapid stream of fresh water which the mother drives through her hole for the purpose of her own respiration.

The accompanying representation of the embryo of Squilla shows that it possesses a long, segmented abdomen without appendages, a bilobate tail, six pairs of limbs, and a short heart; the latter only pulsates weakly and slowly. If it acquires more limbs before exclusion, the youngest larva must stand on the same level as the youngest larva of Euphausia observed by Claus.

(FIGURE 34. Embryo of a Squilla, magnified 45 diam. a. heart.

FIGURE 35. Older larva (Zoea) of a Stomapod, magnified 15 diam.)

Of the two larval forms at present known which are with certainty to be ascribed, if not to Squilla, at least to a Stomapod, I pass over the younger one ( ‘Archiv fur Naturgeschichte’ 1863 Taf 1.) as its limbs cannot be positively interpreted, and will only mention that in it the last three abdominal segments are still destitute of appendages. The older larva (Figure 35), which resembles the mature Squilla especially in the structure of the great raptorial feet and of the preceding pair, still wants the six pairs of feet following the raptorial feet. The corresponding body-segments are already well developed, an unpaired eye is still present, the anterior antennae are already biramose, whilst the flagellum is wanting in the posterior, and the mandibles are destitute of palpi; the four anterior abdominal segments bear biramose natatory feet, without branchiae; the fifth abdominal segment has no appendages, and this is also the case with the tail, which still appears as a simple lamina, fringed on the hinder margin with numerous short teeth. It is evident that the larva stands essentially in the grade of Zoea.

CHAPTER 8. DEVELOPMENTAL HISTORY OF EDRIOPHTHALMA.

Less varied than that of the Stalk-eyed Crustacea is the mode of development of the Isopoda and Amphipoda, which Leach united in the section Edriophthalma, or Crustacea with sessile eyes.

(FIGURE 36. Embryo of Ligia in the egg, magnified 15 diam. D. yelk; L. liver.)

The Rock-Slaters (Ligia) may serve as an example of the development of the Isopoda. In these, as in Mysis, the caudal portion of the embryo is bent not downwards, but upwards; as in Mysis also, a larval membrane is first of all formed, within which the Slater is developed. In Mysis this first larval skin may