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Vol. xv. - SEPTEMBER, 1881. — No. 9.
VARIATIONS IN A COPEPOD CRUSTACEAN.
BY CARL F. GISSLER, PH.D. N March, 1878, a large, deep pond near Glendale, Long Island,
was found densely populated with blood red Crustaceans, which, microscopically examined, proved to be a form slightly aberrant from Diaptomus sanguineus, described by Professor S. A. Forbes," who figured its inferior and superior maxilliped and fifth pair of legs of the male and female.
The Long Island form’ is unquestionably the same species as that from Illinois, but the climate and locality have effected slight morphological changes of those organs which in Copepod Crustaceans are most liable to occur. The differences noted between the individuals from the two localities are, however, very trifling, and its elevation to the rank of a new species would not be advisable. It appears, as in so many other instances, that careful examinations of a species from different localities do not demonstrate the constancy, but the evolution of the same. The changes sometimes concern insignificant structures, but often also the most important parts used by the systematist in describing a species.
To make the study of Copepods3 in general, better understood to the amateur, I have more thoroughly described and figured all their external structures.
The body of Copepoda is more or less distinctly segmented and
1 Bulletin of the Illinois Museum of Natural History. No. 1. List of Crustaceans, with descriptions of new species, by S. A. Forbes, 1876.
2 Measures 3mm. in length.
3 Consult also “ Die freilebenden Copepoden,” by Professor Dr. C. Claus. Leipzig, 1863, page 200.
distinguishable into regions, with two pairs of antennæ, the anterior pair much larger, either one or both of the latter often transformed into an auxiliary, prehensile, copulative organ. They possess neither a carapace nor a bivalve shell; but have three pairs of mouth-parts and five pairs of swimming feet. Females with external egg-sac.
Family of Calanidæ.—Body elongate, similar to that of Pontellidæ, anterior antennæ very long, usually of twenty-four to twenty-five joints. In the male sex the right, rarely the left antenna is transformed into a geniculating, prehensile organ. Posterior antennæ comparatively large, two-branched. Mandibulary palpus two-branched, similar to the posterior antennæ. Maxillæ with a large, manifold-lobed palpus. Maxillipeds powerfully devel
oped. Fifth pair of legs large, either similar to
We now drop some live specimens into alco-
Placing a male specimen (Fig. 4) on a glass slide under the dissecting microscope with low power, we now proceed with the dissecting needles to separate successively the different
appendages, viz., the anterior long antennæ, the Fig. 1.-Left an
shorter posterior antennæ, the mandibles with male.
palpus, the superior maxillipeds, the maxillæ, the inferior maxillipeds, the four pairs of natatory legs, the fifth, trans
formed pair of legs, and finally the abdomen with the terminal furca. Viewing the remaining “carcass” from which those appendages have been taken, we will notice that there is a certain demarcation between head and thorax, forming a segment; following this we find five more thoracic segments, of which the fifth is half as long as either the second or third, the fourth slightly shorter than the second or third, which latter two are equally long, the first being somewhat longer.
In the female the first and fourth thoracic segments are longer than the second and third, the fifth is faintly sub-segmented on the dorsal side, laterally terminating in a strong spine similar to Ichthyophorba denticornis Claus (Opus citatum, p. 199, Tab. xxxv, Fig. 1).
We now place the glass slide under a compound microscope, applying a low magnifying power and inspect the left, normally shaped, anterior antenna (see Fig. 1).
It is beset with pretty large bristles, and consists of twenty-five joints. When connected with the body, the fifteenth joint will reach to the base of the abdomen, and the terminal bristles reach to near the tip of the furcal bristles. The anterior antennæ originate from the first pair of larval legs, and are the means with which the Diaptomus performs its peculiar jerking motions, described by Herrick in AMERICAN NATURALIST, 1879, page 622.
In glancing at the right antenna of the male (see Fig. 2) one would think it had been taken from an entirely different species-so dissimilar it looks! If we count the joints, we find but twenty-three, two joints less than in the left antenna. But either the tenth or twelfth joint must consist of two connate joints, and the twenty-first is evidently also sub-segmented, making in all, twenty-five joints. The sixteenth to the nineteenth joints, inclusive, are considerably dilated and swollen, enclosing a puwerful muscle, inserted near the fifteenth and in the twentieth joint; thence follows the knee-shaped sec
FIG. 2.–Right anterior antion of the antenna, the geniculating part, consisting of a larger joint with an inner duplicature or bead, forming a tier (originally several connate segments), and another larger, semi-segmented joint with a terminal, inner, bentbackward hook, and finally two smaller terminal joints. The dilated joints as well as some of the narrower preceding joints, are
tenna of male.
armed with powerful spines, and others with bristles. If this swollen right antenna be separated from a live male, it will twist around with snake-like motions for several minutes.
As the external structures, with the exception of the fifth thoracic segment, the anterior antennæ, the fifth pair of legs and the abdomen, are alike in both sexes, we may proceed to the posterior or second pair of antennæ (see Fig. 3).
The posterior antennæ originate from the second pair of larval legs and
have like those two branches. They Fig. 3.-—Posterior or second
are destined for locomotion, and antenna. Main branch shorier.
also for respiration. The main branch is slightly shorter than the secondary branch.
Having once with certainty recognized the mandible (Fig. 4), then, after applying higher powers (about 500 X), we are enabled
to see the following characters: The tip of the first (outer) tooth is bent and has a very minute excavation. There are eight mandibular teeth. The second tooth is larger than any of the remaining six of the series, its suddenly contracted tip somewhat bent like the first tooth; it is separated from the third by
an interval equal to the width Fig. 4.—Mandible and mandibulary of the tooth. A short stift palpus. Enlarged about 300 X.
bristle appears at the lower end of the row of teeth. The secondary appendage of the mandibular palpus is four-jointed and bears six delicate bristles at its tip and inner margin. The larger, bent-upward bristle near the inner
base of the main mandibulary palpus is delicately bristled exteriorly only. Three smaller ciliæ are found above the latter on a small protuberance.
The mandible in all Copepoda originates from the third larval leg, which is already in the "Nauplius stage," provided with a dentate mandibular process. In the family of Calanidæ, the mandibulary palpus is comparatively longer than in the other five families of Copepoda.
The maxilla (Fig. 5) is quite a complicated structure, consisting of several lobes, the explanation of which is found below the cut. The maxillæ are the second pair of oral organs, and
Fig. 5.-Maxilla. a, maxilla proper; b, basal broad
lobe; cand d, two cylindrical basal processes; e, terminal originate from the palpus; f, posterior lobe, or secondary branch of palpus. bristled and lobed Enlarged about 300 X. appendages of the larva, and these occur behind the third natatory leg, or future mandibulary palpus.
Another minute mouth-piece is the superior or first maxilliped (Fig. 6). It is somewhat sub-jointed, elongate, and bears fifteen bristles as the illustration shows. Both the superior and inferior maxillipeds are the separately diverging branches of a single pair of limbs originating out of the fifth pair of legs of the later “Nauplius stage,” and are in the adult, with a few exceptions, dissimilarly inserted, the outer branches of those legs being transformed into the superior, the inner branches into the inferior maxillipeds.
Fig. 6.-Superior or first We now take a look at Fig. 7, represent- maxilliped. Enlarged about ing the inferior or second maxilliped. Its 400 X. basal segment presents in our species four rounded processes on