170 



PTEROPODA. 



in an excess of the former, except the acetic, 

 and insoluble in excess of the latter. If a so- 

 lution of casein be allowed to stand some time, 

 lactic acid is gradually formed, which causes 

 it to coagulate, and putrefaction then begins, 

 which, if any sugar is present, determines in 

 it the alcoholic fermentation. 



The various forms of protein which are found 

 constituting the muscles, tissues, and solid 

 matters of the blood of animals, are thus evi- 

 dently derived from the vegetable kingdom ; 

 that silent but ever active laboratory in which 

 so much of the chemical economy of nature is 

 carried on. From the gaseous matters of the 

 atmosphere, more especially carbonic acid, 

 ammonia, and watery vapour, the organic ele- 

 ments, carbon, oxygen, hydrogen, and nitrogen, 

 are derived; and from the various saline 

 ingredients of the soil, those inorganic sub- 

 stances which are essential to the growth and 

 well-being of mankind and of the lower ani- 

 mals are readily abstracted by the absorbent 

 fibres of the roots. Thus formed, plants con- 

 stitute the source from which all living beings 

 obtain the nourishment which is necessary to 

 their existence, and of which the very sub- 

 stance of their bodies is composed ; an arrange- 

 ment which is most strikingly evident in the 

 herbivora, because vegetables are their only 

 food, but not less certainly in the carnivora, 

 since the animal flesh which they consume is 

 either that of the herbivora or of some ani- 

 mals which have fed upon them. 



It is impossible not to admire the simplicity 

 which pervades the whole of this vast scheme, 

 in which we find so large a portion of the 

 animal body composed of materials almost 

 identical in composition, though differing so 

 essentially in their use and applications. If 

 one of these principles, albumen or casein for 

 instance, be contained in the food in quantity 

 insufficient for the requirements of the animal, 

 it is readily supplied from one of the others by 

 the addition or removal of a minute quantity 

 of sulphur or phosphorus, both of which are 

 always present ; whereas, if this beautiful pro- 

 vision had not been made, a large amount of 

 disease and suffering would have almost neces- 

 sarily ensued. Moreover, had the task of ela- 

 borating these highly complex principles from 

 more simple ingredients devolved on ani- 

 mals themselves, much complicated machinery 

 would probably have been required, which 

 would have added unnecessarily to the com- 

 plexity of the body, and consequently to the 

 sources of physical derangement. 



[Since the above has been in type, some 

 researches, which are still in progress, have 

 thrown a doubt upon the exact composition 

 of protein, and indeed rendered it uncertain 

 whether it can be obtained in a state per- 

 fectly free from sulphur.] 



BIBLIOGRAPHY. The following books may be 

 mentioned as containing the fullest descriptions of 

 protein and its compounds, together with other 

 branches of physiological chemistry : Simon, Hand- 

 buch der angcwandten mediziuischeu Chemie, of 



which a translation has been published by the Sy- 

 denham Society. Lielip, Traite de Chimie orga- 

 niquc, torn. i. & iii. Liebig's Animal Chemistry, 

 translated by Gregory. Mulder, Chemistry of vege- 

 table and animal physiology, translated from the 

 Dutch by Fromberg ; and Dumas, Traite' de Chimie 

 appliqueu aux arts, torn. vii. & viii. Besides these 

 many detached papers of great value will be found 

 in the later volumes of the Annales de Chimie et de 

 Physique ; Annalen der Chemie und Pharmacie, by 

 Lieblq and Wohler ; Poggendorjfs Annalen der 

 Phys'ik und Chemie; Philosophical Transactions, 

 Philosophical Magazine, &c. 



(./. E. Bowman.) 



PTEROPODA (Gr. TTT^OV, a wing, TroCs, a 

 foot ; Fr. Pteropodes ; Lat. Mollusca jmi- 

 nata). An order of Molluscous animals es- 

 tablished by Cuvier, and named in accord- 

 ance with his arrangement of the Molluscous 

 division of the animal kingdom, from the po- 

 sition of their organs of locomotion, which 

 in the creatures we are about to examine is 

 very remarkable. All the animals belonging 

 to the order are marine, and in some regions 

 of the ocean crowd the surface of the sea at 

 certain seasons in immense numbers, swim- 

 ming by the aid of two muscular expansions 

 resembling fins, which are attached to the op- 

 posite sides of the neck, and serve as paddles, 

 although, in the language of Natural History, 

 named feet. 



Notwithstanding the multitudes of indi- 

 viduals belonging to this group, which are said 

 to swarm both in the polar regions and in tro- 

 pical climes, the number of genera at present 

 ascertained to exist is very limited, and such 

 is their minute size and the delicacy of their 

 structure, which precludes the possibility of 

 studying them, unless in a fresh state, that, 

 up to a very recent period, their anatomy was 

 imperfectly understood, and, doubtless, much 

 remains yet to be achieved by those who may 

 be favourably situated or investigating them 

 more closely. 



The characters which they present in 

 common, and by which they are separated by 

 naturalists as a distinct group of Mollusca, 

 are the following : Their bodies are free, 

 and organized for natation ; they are fur- 

 nished with a distinct head, but possess no 

 locomotive organs, except a pair of lateral 

 fins. 



Genera. 

 CLIO (fig. 108). 



H YALE A (fig. 114). 



PNEUMODERMA (Jig. 115). 

 CYMBULIA. 



LlMACINA. 



CLEODORA. 

 ATLANTA. 



M. d'Orbigny, in a memoir read before the 

 Academy of Sciences in Paris*, gives some 

 interesting particulars relative to the organi- 

 zation and habits of this remarkable class of 

 molluscous animals. They are met with in 

 all seas, under the equator as well as in the 



* Vide Ann. des Sciences Nat, for 1835, p. 189. 



