573 



SUPPLEMENT. 



ORDER VIII. XYPHOSURA. 



Body in two parts; without a siphon; base of the 

 feet, except the last, spinous, which serve as jaws; with 

 a hard twin shell covering the whole body, haying 

 longitudinal furrows above; termination of the body 

 sabre-shaped. This order consists of but one species, 

 Liinulus Polyphemus. 



ORDER IX. SIPHONOSTOMA. 

 FAMILY I. CALIGIDES. 



Body somewhat oval, with a distinct shell. The 

 genera are Ar</uhis, Caligus, Cecrops, and Antfiosoma 

 Smilhii, pi. 23, f. 25. 



FAMILY II. LERN^IFORMES. 



Destitute of shell; body nearly cylindrical, jointed, 

 and worm-shaped. Thisfamily consists but of one species, 

 Dichelesthium Sturionis. Many fossil species hare been 

 found, which naturalists refer to existing genera. 



ANATOMY OF A PLAGUSIA. Order I. Family I. PI. 

 30, f. 3 1 , head : a , a, eyes ; f. 32, an eye seated on its pedi- 

 cle; f. 35, tail; f. 29 and 34, teeth; f. 38 and 30, antennas; 

 f. 27, post abdomen; f. 26, internal structure of thorax; 

 f. 33, internal structure of abdomen. 



MOLLUSCOUS ANIMALS. 



MOLLUSCOUS ANIMALS, like Crustacea, are com- 

 prehended in the second great division of the 

 animal kingdom animals without a back- 

 bone. They are destitute of an articulated 

 skeleton or vertebral column. The nervous 

 system is not developed in the form of a spi- 

 nal cord, but simply into a certain number of 

 medulary masses situated in different points 

 of the body, the principal of which is called 

 the brain, and is placed transversely on the 

 oesophagus, and envelopes it with a nervous 

 collar. The organs of sensation and motion 

 have not the same uniformity in point of num- 

 ber and position as in the vertebrate animals, 

 and a greater aberration is observable in the 

 position of the heart and organs of respiration, 

 as well as in the structure of the latter. Some 

 species are formed for breathing elastic air, 

 and others fresh or salt water. Their organs 

 of locomotion, and others, which are external, 

 are generally symmetrically arranged on the 

 two sides of an axis. 



The mollusca have a double circulation, 

 their pulmonary system invariably describing 

 a distinct circle. The function of breathing 

 is always assisted by, at least, one ventrical, 

 situated between the pulmonary veins and the 

 arteries of the body and not, as in fishes, be- 

 tween the veins of the body and pulmonary 

 arteries. It is then an aortic ventrical. It 

 is only the cephalopoda 1 that are provided 

 with a pulmonary ventrical, which is subdi- 

 vided. The aortic ventrical is likewise divided 

 in some genera, as in the area 2 and lingu- 

 li. 3 In some other bivalves, the auricle only 



1 See note on Conchology, in the present volume, 

 Class I., Order II., p. 354. 



8 Ib. Class II., Order II., p. 355. 

 8 Ib. Older I., p. 355. 



is divided. When more than one ventrical 

 exists, they do not consist of a single mass, 

 as in warm-blooded animals, but are remote 

 from each other. 



The blood in molluscous animals is white, 

 or bluish- white, and seems to contain a smaller 

 proportion of fibrin than that of vertebrate 

 animals. Cuvier supposes that the veins per- 

 form the functions of absorbent vessels. 



The muscles in this class are attached to 

 their skin by various points, forming, in those 

 places, tissues which possess more or less den- 

 sitv. Their motions consist of different con- 

 tractions, varying in their direction, produc- 

 ing inflections and prolongations, together 

 with relaxations of their several parts, by 

 means of which they creep, swim, and seize 

 upon such objects as the formation of these 

 parts are adapted to. They are, however, in- 

 capable of rapid progress, their limbs not being 

 supported by articulated and solid levers. 



Most of the mollusca are possessed of great 

 irritability, frequently continuing after they 

 are cut asunder. Their skin is naked, ex- 

 tremely sensible, and usually covered with a 

 mucous substance, which is secreted from its 

 pores. No organ of smell has yet been de- 

 tected in them, although they appear to pos- 

 sess that sense. Cuvier thinks it probable 

 that the whole skin may be susceptible of dis- 

 tinguishing odours. All the cephala.brachio- 

 poda, cirripeda, and part of the gasteropoda, 

 and pteropoda are destitute of eyes, while ths 

 cephalopoda enjoy them in as complicated a 

 degree as the warm-blooded animals. The 

 eyes are situated sometimes at the base, some- 

 times at the middle, and frequently at the ex- 

 tremity of the tentacula. The cephalopoda 

 are the only ones in which have been disco- 

 vered the organ of hearing, and where the 

 brain is enveloped in a kind of case, of a car- 

 tilaginous consistence. 



Almost all molluscous animals have a pe- 

 culiar development of the skin, which covers 

 their body, like a mantle, and has received 

 that appellation. This process, however, is 

 sometimes narrowed into a simple disk, or is 

 tubiform, or hollowed into a sac, and in some 

 cases it is divided and extended in the shape of 

 fins. There are two distinct kinds of mollus- 

 cous animals, namely, cephalous, or such as 

 are provided with a head, and acephalous, or 

 destitute of a head. These are again distin- 

 guished by such as are naked mollusca and 

 testaceous mollusca, or with a shelly envelop. 

 The naked mollusca have a membranous or 

 fleshy mantle ; which, however, has frequently 

 one or more hard laminae in its texture. These 

 are increased in dimensions and thickness, by 

 a deposit of earthy and animal matter com- 

 bined ; the new layers overlapping the old 

 ones. When this substance remains concealed 



