366 
connecting the Mollusks with the Zoophytes, 
it affords perhaps the best positive character of 
this great primary group of the animal king- 
dom; for whereas, in the Articulate division, 
the breathing organs are lateral or open upon 
or towards the sides of the body, and in the 
Vertebrate division communicate with the oral 
extremity of the nutritive canal, in the Mollusca 
they are connected with the anal outlet. 
 uropoietic system, where traces of it are 
oo e, as in most Gastropods and in 
Cephalopods, likewise communicates withthe 
respiratory cavity. 
e rich endowment of vibratile cilia de- 
Serves to be noticed as characterizing the bran- 
chial organs, and constituting the chief mecha- 
nism 1 gebete in most Mollusca. The 
organs of vegetative life subservient to the pro- 
creation of the species are not less remarkable 
for bulk, variety, and complexity than are those 
which minister to the preservation and growth 
of the individual. Although comparatively 
simple and reduced to the essential formative 
organs in the Acephala, they are, with very 
few exceptions, placed in distinct individuals, 
that is to say, one Ascidian or Oyster possesses 
only the testicle, and is a male; another only 
the ovarium, and is a female. In the order 
Conchifera, the females have the gills modified 
to serve as a receptacle for the impregnated ova 
during foetal development. 
Among the Encephalous Mollusks are many 
hermaphroditical species, some with the male 
and female organs terminating sufficiently close 
together for independent or self-impregnation; 
others having the outlets of the two organs 
remote, and requiring the concourse of another 
individual in reciprocal fecundation ; or the 
same individual is impregnated by another and 
supeapuater a third, as is curiously exempli- 
fied in the nuptial chain thus formed by nu- 
merous individuals of the Marsh-snails ( Lym- 
n@a). The Trachelipods and Cephalopods 
are again diecious, like the lowest classes 
of Mollusks, but exhibit the generative organs 
under the highest stage of complication; and 
some of the latter class are remarkable for 
the expulsion not only of the ova aggregated 
in groups contained in special receptacles, but 
also of the spermatozoa in a similar state of 
aggregation in cylindrical cases, which, by the 
arrangement of their elastic tissue, manifest 
movements, prior to their rupture, which have 
long excited the surprise and admiration of the 
physiological observer. 
A large vitellus, among the numerous nucle- 
ated cells of which it is often difficult to recog- 
nize a single one as the centre of development, 
or as the germinal vesicle, characterizes the ovum 
of most Mollusks. In most species also the early 
formation of vibratile cilia on the surface of the 
germinal membrane, and the rotation of the 
embryo upon its axis produced by their action, 
are striking though not peculiar phenomena. The 
adherence to a uniform type in the earlier pe- 
riods of growth is singularly manifested in the 
rr of Tritonia, Doris, Aplysia, and other 
naked Mollusks, which are protected for a cer- 
tain period by an external spiral univalve shell. 
MONOTREMATA. 
The classification of the Molluscous vine 
has exercised the judgment and discrimination 
of some of the ablest Zoologists, and is a sub- 
ject too expanded for the limits assigned to the 
present article. The principles of a natural 
distribution into the larger groups according 
to general organization seem to be adhered to 
in the following system. ay ; 
Taking the nervous system as a guide to 
the divisions of highest value and extent, the 
Mollusca separate themselves, as already shown, 
into AcepHata and EycEPHALa. ‘ 
The Acephala may be divided to 
the nature of their external covering into Tu- 
nicaTa, where this is continuous, flexible, and 
elastic; and into Concutrera, where it is tes- 
taceous and divided into two or more valves, 
The Conchifera may be subdivided according 
to the modifications of the respiratory cet 
into Palliobranchians and 7 
Respiratory characters likewise mainly distin- 
guish the sessile Tunicaries or Ascidians, and 
the floating Tunicaries, or Salpaceans. — " 
The Encephalous Mollusks are classified ac- 
cording to their organs of locomotion, as Pre- 
ropops, Gasrropops, and CEPHALopops. 
The respiratory organs afford the best charaec- 
ters for the subdivisions or orders of these three 
classes. , 
( Richard Owen.) 
MONOTREMATA, Gr. novos, single, rence, 
hole, in reference to the single cl excre- 
mentory and generative outlet; Fr. Mono- 
trémes ; Ger. Monotremen ; Eng. 
An order or primary group of the Implacental 
subclass of Mamma ia, representin Eden- 
tata in that subclass; allied to the ialia 
by the absence of the corpus callosum and by 
the presence of the marsupial bones, but differ- 
ing in the absence of the abdominal pouch and 
scrotum, in the absence of teeth, in the sim- 
plicity of the bigeminal bodies, and in some 
remarkable modifications of the skeleton and 
generative organs. 
As the order Bruta or Edentata is that which 
exhibits the lowest modifications of the Pla- 
cental type of the Mammalian structure, and 
offers in some respects the nearest approach in 
that subclass to the Ovipara, so the Monotre- 
mata present the extreme modifications of the 
Implacental type, and make the last step in the 
transition from the Mammalian to the Oviparous — 
classes. 
The Monotremes are, however, trae Mam- 
malia in all essential points of structure: th 
possess functional mammary glands, which 
are largely developed at the breeding season ; 
their lungs consist of a spongy tissue, sub- 
divided throughout into very minute cells; 
they are suspended freely in a thoracic cavity, 
separated by a complete muscular and aponeu- 
rotic diaphragm from the abdomen: the arch 
of the aorta bends over the left bronchus: the 
larynx is superior, and is defended by a well- 
developed epiglottis : the kidneys are compact 
conglobate glands with distinct cortical and 
medullary substances, secreting the urine from 
arterial blood, and returning the blood to the 
