1. _Cunanthidae._--With broad gastric pouches which are simple, i.e.
undivided, and "pernemal," i.e. correspond in position with the
tentacles. _Cunina_ (fig. 66) with more than eight tentacles;
_Cunoctantha_ with eight tentacles, four perradial, four interradial.
2. _Aeginidae._--Radii a multiple of four, with radial gastric pouches
bifurcated or subdivided; the tentacles are implanted in the notch
between the two subdivisions of each (primary) gastric pouch, hence
the (secondary) gastric pouches appear to be "internemal" in position,
i.e. to alternate in position with the tentacles. _Aegina_, with four
tentacles and eight pouches; _Aeginura_ (fig. 25), with eight
tentacles and sixteen pouches; _Solmundella_ (fig. 67), with two
tentacles and eight pouches; _Aeginopsis_ (fig. 23), with two or four
tentacles and sixteen pouches.
3. _Solmaridae._--No gastric pouches; the numerous tentacles arise
direct from the stomach, into which also the peronial canals open, so
that the ring-canal is cut up into separate festoons. _Solmaris_,
_Pegantha_, _Polyxenia_, &c. To this family should be referred,
probably, the genus _Hydroctena_, described by C. Dawydov [11a] and
regarded by him as intermediate between Hydromedusae and Ctenophora.
See O. Maas [35].
_Appendix to the Trachylinae._
Of doubtful position, but commonly referred to the Trachylinae, are
the two genera of fresh-water medusae, _Limnocodium_ and _Limnocnida_.
_Limnocodium sowerbyi_ was first discovered in the _Victoria regia_
tank in the Botanic Gardens, Regent's Park, London. Since then it has
been discovered in other botanic gardens in various parts of Europe,
its two most recent appearances being at Lyons (1901) and Munich
(1905), occurring always in tanks in which the _Victoria regia_ is
cultivated, a fact which indicates that tropical South America is its
original habitat. In the same tanks a small hydroid, very similar to
_Microhydra_, has been found, which bears medusa-buds and is probably
the stock from which the medusa is budded. It is a remarkable fact
that all specimens of _Limnocodium_ hitherto seen have been males; it
may be inferred from this either that only one polyp-stock has been
introduced into Europe, from which all the medusae seen hitherto have
been budded, or perhaps that the female medusa is a sessile gonophore,
as in _Pennaria_. The male gonads are carried on the radial canals.
_Limnocnida tanganyicae_ was discovered first in Lake Tanganyika, but
has since been discovered also in Lake Victoria and in the river
Niger. It differs from _Limnocodium_ in having practically no
manubrium but a wide mouth two-thirds the diameter of the umbrella
across. It buds medusae from the margin of the mouth in May and June,
and in August and September the gonads are formed in the place where
the buds arose. The hydroid phase, if any, is not known.
Both these medusae have sense-organs of a peculiar type, which are
said to contain an endodermal axis like the sense-organs of
Trachylinae, but the fact has recently been called in question for
_Limnocodium_ by S. Goto, who considers the genus to be allied to
_Olindias_. Allman, on the other hand, referred _Limnocodium_ to the
Leptomedusae.
In this connexion must be mentioned, finally, the medusae budded from
the fresh-water polyp _Microhydra_. The polyp-stages of _Limnocodium_
and _Microhydra_ are extremely similar in character. In both cases the
hydranth is extremely reduced and has no tentacles, and the polyp
forms a colony by budding from the base. In _Limnocodium_ the body
secretes a gelatinous mucus to which adhere particles of mud, &c.,
forming a protective covering. In _Microhydra_ no such protecting case
is formed. In view of the great resemblance between _Microhydra_ and
the polyp of _Limnocodium_, it might be expected that the medusae to
which they give origin would also be similar. As yet, however, the
medusa of _Microhydra_ has only been seen in an immature condition,
but it shows some well-marked differences from _Limnocodium_,
especially in the structure of the tentacles, which furnish useful
characters for distinguishing species amongst medusae. The possession
of a polyp-stage by _Limnocodium_ and _Microhydra_ furnishes an
argument against placing them in the Trachylinae. Their sense-organs
require renewed investigations. (Browne [10] and [10a].)
ORDER VI. Siphonophora.--Pelagic floating Hydrozoa with great
differentiation of parts, each performing a special function; generally
regarded as colonies showing differentiation of individuals in
correspondence with a physiological division of labour.
[Illustration: FIG. 68.--Diagram showing possible modifications of
medusiform and hydriform persons of a colony of _Siphonophora_. The
thick black line represents endoderm, the thinner line ectoderm. (After
Allman.)
n, Pneumatocyst.
k, Nectocalyces (swimming bells).
l, Hydrophyllium (covering-piece).
i, Generative medusiform person.
g, Palpon with attached palpacle, h.
e, Siphon with branched grappling tentacle, f.
m, Stem.]
A typical Siphonophore is a stock or _cormus_ consisting of a number of
_appendages_ placed in organic connexion with one another by means of a
_coenosarc_. The coenosarc does not differ in structure from that
already described in colonial Hydrozoa. It consists of a hollow tube, or
tubes, of which the wall is made up of the two body-layers, ectoderm and
endoderm, and the cavity is a continuation of the digestive cavities of
the nutritive and other appendages, i.e. of the coelenteron. The
coenosarc may consist of a single elongated tube or stolon, forming the
stem or axis of the cormus on which, usually, the appendages are
arranged in groups termed cormidia; or it may take the form of a compact
mass of ramifying, anastomosing tubes, in which case the cormus as a
whole has a compact form and _cormidia_ are not distinguishable. In the
Disconectae the coenosarc forms a spongy mass, the "_centradenia_,"
which is partly hepatic in function, forming the so-called liver, and
partly excretory.
The appendages show various types of form and structure corresponding to
different functions. The cormus is always differentiated into two parts;
an upper portion termed the _nectosome_, in which the appendages are
locomotor or hydrostatic in function, that is to say, serve for swimming
or floating; and a lower portion termed the _siphosome_, bearing
appendages which are nutritive, reproductive or simply protective in
function.
Divergent views have been held by different authors both as regards the
nature of the cormus as a whole, and as regards the homologies of the
different types of appendages borne by it.
The general theories of Siphonophoran morphology are discussed below,
but in enumerating the various types of appendages it is convenient to
discuss their morphological interpretation at the same time.
[Illustration: After A. Agassiz, from Lankester's _Treatise on
Zoology_.
FIG. 69.--_Porpita_, seen from above, showing the pneumatophore and
expanded palpons.]
In the nectosome one or more of the following types of appendage
occur:--
1. Swimming-bells, termed _nectocalyces_ or _nectophores_ (fig. 68,
k), absent in _Chondrophorida_ and _Cystophorida_; they are
contractile and resemble, both in appearance, structure and function,
the umbrella of a medusa, with radial canals, ring-canal and velum;
but they are without manubrium, tentacles or sense-organs, and are
always bilaterally symmetrical, a peculiarity of form related with the
fact that they are attached on one side to the stem. A given cormus
may bear one or several nectocalyces, and by their contractions they
propel the colony slowly along, like so many medusae harnessed
together. In cases where the cormus has no pneumatophore the topmost
swimming bell may contain an oil-reservoir or _oleocyst_.
2. The pneumatophore or air-bladder (fig. 68, n), for passive
locomotion, forming a float which keeps the cormus at or near the
surface of the water. The pneumatophore arises from the ectoderm as a
pit or invagination, part of which forms a gas-secreting gland, while
the rest gives rise to an air-sack lined by a chitinous cuticle. The
orifice of invagination forms a pore which may be closed up or may
form a protruding duct or funnel. As in the analogous swim-bladder of
fishes, the gas in the pneumatophore can be secreted or absorbed,
whereby the specific gravity of the body can be diminished or
increased, so as to cause it to float nearer the surface or at a
deeper level. Never more than one pneumatophore is found in a cormus,
and when present it is always situated at the highest point above the
swimming bells, if these are present also. In _Velella_ the
pneumatophore becomes of complex structure and sends air-tubes, lined
by a chitin and resembling tracheae, down into the compact coenosarc,
thus evidently serving a respiratory as well as a hydrostatic
function.
Divergent views have been held as to the morphological significance of
the pneumatophore. E. Haeckel regarded the whole structure as a
glandular ectodermal pit formed on the ex-umbral surface of a
medusa-person. C. Chun and, more recently, R. Woltereck [59], on the
other hand, have shown that the ectodermal pit which gives rise to the
pneumatophore represents an entocodon. Hence the cavity of the
air-sack is equivalent to a sub-umbral cavity in which no manubrium is
formed, and the pore or orifice of invagination would represent the
margin of the umbrella. In the wall of the sack is a double layer of
endoderm, the space between which is a continuation of the
coelenteron. By coalescence of the endoderm-layers, the coelenteron
may be reduced to vessels, usually eight in number, opening into a
ring-sinus surrounding the pore. Thus the disposition of the
endoderm-cavities is roughly comparable to the gastrovascular system
of a medusa.
The difference between the theories of Haeckel and Chun is connected
with a further divergence in the interpretation of the stem or axis of
the cormus. Haeckel regards it as the equivalent of the manubrium, and
as it is implanted on the blind end of the pneumatophore, such a view
leads necessarily to the air-sack and gland being a development on the
ex-umbral surface of the medusa-person. Chun and Woltereck, on the
other hand, regard the stem as a _stolo prolifer_ arising from the
aboral pole, that is to say, from the ex-umbrella, similar to that
which grows out from the ex-umbral surface of the embryo of the
Narcomedusae and produces buds, a view which is certainly supported by
the embryological evidence to be adduced shortly.