706 



CONCHIFERA. 



branchise are included. It is in this palleal 

 sac that the animal establishes a current of 

 water, destined to minister to the function of 

 respiration, and to carry towards the mouth the 

 alimentary particles with which it is fed. The 

 median parts of the lobes of the mantle are ex- 

 tremely thin and transparent, and a great 

 number of vessels (c, Jig. 362), and a few 

 nervous filaments (7, Q, Jig. 360) are perceived 

 ramifying through their substance, and running 

 towards the anterior and inferior edges. These 

 edges, which extend as far as those of the shell, 

 are thickened, and it is at the point where the 

 thickening begins that the mantle adheres to the 

 shell by means of a great number of minute 

 muscles (/, I, Jig. 347; d,fg. 362), which leave 

 a linear impression upon it. The thickening of 

 the edges of the mantle is owing to the pre- 

 sence of a great quantity of muscular fibres, fre- 

 quently to several rows of contractile tentacular 

 cilia (m, m, Jig. 347; e, Jig. 361 &362); and, 

 lastly, to that of an organ, which is the secerning 

 apparatus of the shell. The muscular fibres are 



Fig.ZQl. 



Contractile cilia magnified. 



distributed some to the edges of the mantle, 

 and others to the tentacula with which it is 

 fringed. The whole of these parts are extremely 

 retractile, and are endowed with such sensi- 

 bility that the slightest contact is perceived, as 

 is evinced by their instantaneous contraction. 



Zoologists have taken advantage of certain 

 modifications in the lobes of the mantle to 

 establish divisions in their methodical arrange- 

 ments of the conchifera. This artificial means 

 is sufficiently convenient, inasmuch as no 

 anatomical inquiries are necessary in order to 

 get at the distinguishing characters which these 

 modifications supply. Latreille, in his ' Fa- 

 milies du Regne Animal/ as well as other 

 zoologists, have also made use of the conjunc- 

 tion or disunion of the lobes of the mantle to 

 establish the principal divisions of their classifi- 

 cation ; but they have perhaps given too much 

 consequence to these characters, inasmuch as 

 they bear no relation to the number of the 

 muscles. Nevertheless, none of the Mono- 

 myaria has yet been found which presents 

 the lobes of the mantle conjoined, whilst the 

 Dimyaria exhibit the two modifications which 

 we have had occasion to mention, and which 

 gives an opportunity to divide them into two 

 grand series, the first comprising the whole of 

 the Dimyaria whose mantles are united, the 

 second all those whose mantles are open, or 



unconnected one lobe with another. The con- 

 chiferous Dimyaria which exhibit the lobes of 

 the mantle united are modified in this respect 

 in a remarkable manner, a circumstance which 

 induces us to enter somewhat in detail into 

 this part of the anatomy of the conchifera. 



In making the series of acephalous mol- 

 lusca commence with those which have the 

 lobes of the mantle completely distinct, we 

 may place near them certain genera in which 

 the branchiae, conjoined in their posterior parts, 

 form a kind of canal, within which the anus 

 proceeds to terminate. This conjunction of the 

 branchiae, extending as far as the edge of the 

 mantle, forms a kind of band towards the pos- 

 terior commissure; but, notwithstanding this, 

 it may still be said that these animals have the 

 lobes of the mantle altogether unconnected 

 (Unio) (Jig. 360) ; in other genera which have 

 been held allied to this, the posterior band is 

 not found, and already the lobes of the mantle 

 appear united in the posterior part, to a very 

 small extent, leaving a particular perforation 

 for the anus. The mantle still continues open 

 in its circumference (Mytilus). By-and-by 

 neighbouring genera, and even particular spe- 

 cies of the same genus, instead of a single per- 

 foration, present two (/, g, Jig. 362) ; the 

 second is destined to carry the water directly 

 upon the branchise. When these two perfo- 

 rations have the faculty of being projected 

 beyond the shell in the form of fleshy and con- 

 tractile tubes of various lengths, they have re- 

 ceived the special denomination of siphons; 

 and the term perforation has been reserved to 

 be applied to the holes of the mantle, which 

 never pass the edges of the shell. 



When the two siphons begin to appear, the 

 lobes of the mantle still continue disjoined in a 

 portion of their circumference; and this opening 

 (b, b, fig. 356, h, Jig. 362), is destined for the 

 passage of the foot. 



Fig. 362. 



In proportion as the foot is modified in its 

 form, in proportion as it becomes more rudi- 

 mentary, the two lobes of the mantle are ob- 

 served in the succession of genera to become 

 more and more extensively united, and it hap- 

 pens at length that in certain genera (Mya, 

 Saxicava, &c.) a very minute submedian or 

 anterior perforation, corresponding to the rudi- 

 mentary foot, is all that remains of separation 



