366 



MONOTREMATA. 



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. 



The uropoietic system, where traces of it are 

 recognizable, as in most Gastropods and in 

 Cephalopods, likewise communicates with the 

 respiratory cavity. 



The rich endowment of vibratile cilia de- 

 serves to be noticed as characterizing the bran- 

 chial organs, and constituting the chief mecha- 

 nism of respiration 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 

 impregnates a third, as is curiously exempli- 

 fied in the nuptial chain thus formed by nu- 

 merous individuals of the Marsh-snails ( Lym- 

 naa). The Trachelipods and Cephalopods 

 are again dioecious, 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 

 young of Triton ia, Doris, Aplysia, and other 

 naked Mollusks, which are protected for a cer- 

 tain period by an external spiral univalve shell. 



The classification of the Molluscous animals 

 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. 



Taking the nervous system as a guide to 

 the divisions of highest value and extent, the 

 Mollusca separate themselves, as already shown, 

 into ACEPHALA and ENCEPHALA. 



The Acephala may be divided according to 

 the nature of their external covering into Tu- 

 NICATA, where this is continuous, flexible, and 

 elastic; and into CONCHIFERA, where it is tes- 

 taceous and divided into two or more valves. 

 The Conchifera may be subdivided according 

 to the modifications of the respiratory organs 

 into Palliobranchians and Lamellibranchians. 

 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 PTE- 

 ROPODS, GASTROPODS, and CEPHALOPODS. 

 The respiratory organs afford the best charac- 

 ters for the subdivisions or orders of these three 

 classes. 



(Richard Owen.) 



hole, in reference to the single cloacal excre- 

 mentory and generative outlet ; Fr. Mono- 

 tremes ; Ger. Monotremen ; Eng. Monotremes. 



An order or primary group of the Implacental 

 subclass of MAMMALIA, representing the Eden- 

 tata in that subclass; allied lotheMarsupialia 

 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- 

 muta 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, true Mam- 

 malia in all essential points of structure : they 

 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 



