STRUCTURE OF THE HEART IN THE EDRIOPHTHALMA.
Scarcely less striking than the example of the air-breathing Crabs, is
the behaviour of the heart in the great section Edriophthalma, which
may advantageously be divided, after the example of Dana and Spence
Bate, only into two orders, the Amphipoda and the Isopoda.
In the Amphipoda, to which the above-mentioned naturalists correctly
refer the Caprellidæ and Cyamidæ (Latreille’s _Læmodipoda_), the heart
has always the same position; it extends in the form of a long tube
through the six segments following the head, and has three pairs of
fissures, furnished with valves, for the entrance of the blood,
situated in the second, third, and fourth of these segments. It was
found to be of this structure by La Valette in _Niphargus_ (_Gammarus
puteanus_), and by Claus in _Phronima_; and I have found it to be the
same in a considerable number of species belonging to the most
different families.[1]
The sole unimportant exception which I have hitherto met with is
presented by the genus _Brachyscelus,_[2] in which the heart possesses
only two pairs of fissures, as it extends forward only into the second
body-segment, and is destitute of the pair of fissures situated in this
segment in other forms.[3]
Considering this uniformity presented by the heart in the entire order
of the Amphipoda, it cannot but seem very remarkable, that in the very
next order of the Isopoda, we find it to be one of the most changeable
organs.
In the cheliferous Isopods (_Tanais_) the heart resembles that of the
Amphipoda in its elongated tubular form, as well as in the number and
position of the fissures, but with this difference, that the two
fissures of each pair do not lie directly opposite each other.
Fig. 14. Heart of a young Cassidina. Fig. 15. Heart of a young
Anilocra. Fig. 16. Abdomen of the male of Entoniscus Cancrorum. h.
Heart. l. Liver. Fig. 14. Heart of a young _Cassidina._ Fig. 15. Heart
of a young _Anilocra._ Fig. 16. Abdomen of the male of _Entoniscus
Cancrorum. h._ Heart. _l._ Liver.
In all other Isopoda the heart is removed towards the abdomen. In the
wonderfully deformed parasitic Isopods of the _Porcellanæ_ (_Entoniscus
porcellanæ_), the spherical heart of the female is confined to a short
space of the elongated first abdominal segment, and seems to possess
only a single pair of fissures. In the male of _Entoniscus Cancrorum_
(n. sp.), the heart (Fig. 16) is situated in the third abdominal
segment. In the _Cassidinæ,_ the heart (Fig. 14) is likewise short and
furnished with two pairs of fissures, situated in the last segment of
the thorax and the first segment of the abdomen. Lastly, in a young
_Anilocra,_ I find the heart (Fig. 15) extending through the whole
length of the abdomen and furnished with four (or five?) fissures,
which are not placed in pairs but alternately to the right and left in
successive segments. In other animals of this order, which I have as
yet only cursorily examined, further differences will no doubt occur.
But why, in two orders so nearly allied to each other, should we find
in the one such a constancy, in the other such a variability, of the
same highly important organ? From the schoolmen we need expect no
explanation, they will either decline the discussion of the “wherefore”
as foreign to their province, as lying beyond the boundaries of Natural
History, or seek to put down the importunate question by means of a
sounding paraphrase of the facts, abundantly sprinkled with Greek
words. As I have unfortunately forgotten my Greek, the second way out
of the difficulty is closed to me; but as I luckily reckon myself not
amongst the incorporated masters, but, to use Baron von Liebig’s
expression, amongst the “promenaders on the outskirts of Natural
History,” this affected hesitation of the schoolmen cannot dissuade me
from seeking an answer, which indeed presents itself most naturally
from Darwin’s point of view.
As not only the _Tanaides_ (which reasons elsewhere stated (_vide
suprà_) justify us in regarding as particularly nearly related to the
primitive Isopod) and the Amphipoda, but also the Decapod Crustacea,
possess a heart with three pairs of fissures essentially in the same
position; and as the same position of the heart recurs (_vide infrà_)
even in the embryos of the Mantis-Shrimps (_Squilla_), in which the
heart of the adult animal, and even, as I have elsewhere shown, that of
the larvæ when still far from maturity, extends in the form of a long
tube with numerous openings far into the abdomen, we must
unhesitatingly regard the heart of the Amphipoda as the primitive form
of that organ in the Edriophthalma. As, moreover, in these animals the
blood flows from the respiratory organs to the heart without vessels,
it is very easy to see how advantageous it must be to them to have
these organs as much approximated as possible. We have reason to regard
as the primitive mode of respiration, that occurring in _Tanais_ (_vide
suprà_). Now, where, as in the majority of the Isopoda, branchiæ were
developed upon the abdomen, the position and structure of the heart
underwent a change, as it approached them more nearly, but without the
reproduction of a common plan for these earlier modes of structure,
either because this transformation of the heart took place only after
the division of the primary form into subordinate groups, or because,
at least at the time of this division, the varying heart had not yet
become fixed in any new form. Where, on the contrary, respiration
remained with the anterior part of the body,—whether in the primitive
fashion of Zoëa, as in the _Tanaides,_ or by the development of
branchiæ on the thorax, as in the Amphipoda,—the primitive form of the
heart was inherited unchanged, because any variations which might make
their appearance were rather injurious than advantageous, and
disappeared again immediately.
I close this series of isolated examples with an observation which
indeed only half belongs to the province of the Crustacea to which
these pages ought to be confined, and which also has no further
connexion with the preceding circumstances than that of being an
“intelligible and intelligence-bringing fact” only from the point of
view of Darwin’s theory. To-day as I was opening a specimen of _Lepas
anatifera_ in order to compare the animal with the description in
Darwin’s ‘Monograph on the Subclass Cirripedia,’ I found in the shell
of this Cirripede, a blood-red Annelide, with a short, flat body, about
half an inch long and two lines in breadth, with twenty-five
body-segments, and without projecting setigerous tubercles or jointed
cirri. The small cephalic lobe bore four eyes and five tentacles; each
body-segment had on each side at the margin a tuft of simple setæ
directed obliquely upwards, and at some distance from this, upon the
ventral surface, a group of thicker setæ with a strongly uncinate
bidentate apex. There was above _each_ of the lateral tufts of bristles
a branchia, simple on a few of the foremost segments, and then strongly
arborescent to the end of the body. The animal, a female filled with
ova, evidently, from these characters, belongs to the family of the
Amphinomidæ; the only family the members of which, being excellent
swimmers, live in the open sea.
That this animal had not strayed accidentally into the _Lepas,_ but
appertained to it as a regular and permanent guest, is evidenced by its
considerable size in proportion to the narrow entrance of the test of
the _Lepas,_ by the complete absence of the iridescence which usually
distinguishes the skin of free Annelides and especially of the
Amphinomidæ, by the formation and position of the inferior setæ, etc.
But that a worm belonging to this particular family Amphinomidæ living
in the high sea, occurs as a guest in the _Lepas,_ which also floats in
the sea attached to wood, etc., is at once intelligible from the
stand-point of the Darwinian theory, whilst the relationship of this
parasite to the free-living worms of the open sea remains perfectly
unintelligible under the supposition that it was independently created
for dwelling in the _Lepas._
But however favourable the examples hitherto referred to may be for
Darwin, the objection may be raised against them, and that with perfect
justice, that they are only isolated facts, which, when the
considerations founded upon them are carried far beyond what is
immediately given, may only too easily lead us from the right path,
with the deceptive glimmer of an _ignis fatuus._ The higher the
structure to be raised, the wider must be the assuring base of
well-sifted facts.
Let us turn then to a wider field, that of the developmental history of
the Crustacea, upon which science has already brought together a varied
abundance of remarkable facts, which, however, have remained a barren
accumulation of unmanageable raw-material, and let us see how, under
Darwin’s hand, these scattered stones unite to form a well-jointed
structure, in which everything, bearing and being borne, finds its
significant place. Under Darwin’s hand! for I shall have nothing to do
except just to place the building stones in the position which his
theory indicates for them. “When kings build, the carters have to
work.”
[1] The young animals in the egg, a little before their exclusion, are
usually particularly convenient for the observation of the fissures in
the heart; they are generally sufficiently transparent, the movements
of the heart are less violent than at a later period, and they lie
still even without the pressure of a glass cover. Considering the
common opinion as to the distribution of the Amphipoda, namely, that
they increase in multiplicity towards the poles, and diminish towards
the equator, it may seem strange that I speak of a considerable number
of species on a subtropical coast. I therefore remark that in a few
months and without examining any depths inaccessible from the shore, I
obtained 38 different species, of which 34 are new, which, with the
previously known species (principally described by Dana) gives 60
Brazilian Amphipoda, whilst Kröyer in his ‘Grönlands Amfipoder’ was
acquainted with only 28 species, including 2 Læmodipoda, from the
Arctic Seas, although these had been investigated by a far greater
number of Naturalists.
[2] According to Milne-Edwards’ arrangement the females of this genus
would belong to the “Hypérines ordinaires” and the previously unknown
males to the “Hypérines anormales,” the distinguishing character of
which, namely the curiously zigzagged inferior antennæ, is only a
sexual peculiarity of the male animals. In systematising from single
dead specimens, as to the sex, age, etc. of which nothing is known,
similar errors are unavoidable. Thus, in order to give another example
of very recent date, a celebrated Ichthyologist, Bleeker, has lately
distinguished two groups of the Cyprinodontes as follows: some, the
Cyprinodontini, have a “pinna analis non elongata,” and the others,
the Aplocheilini, a “pinna analis elongata”: according to this the
female of a little fish which is very abundant here would belong to
the first, and the male to the second group. Such mistakes, as already
stated, are unavoidable by the “dry-skin” philosopher, and therefore
excusable; but they nevertheless prove in how random a fashion the
present systematic zoology frequently goes on, without principles or
sure foundations, and how much it is in want of the infallible
touchstone for the value of the different characters, which Darwin’s
theory promises to furnish.
[3] I find, in Milne-Edwards’ ‘Leçons sur la Physiol. et l’Anat.
comp.’ 3 page 197, the statement that, according to Frey and Leuckart,
the heart of _Caprella linearis_ possesses _five_ pairs of fissures. I
have examined perfectly transparent young Caprellæ (probably the young
of Caprella attenuata, Dana, with which they occurred), but can only
find the usual three pairs.