COMTARA 1 Hi: ANATOMY. 



MO 



ganglion being situated above and in front of the stomach. The 



to winch it .-.-.I, M. rvos is very complicated. 

 the only system in which tho liroohiopods seem to have 

 a more comp uoturo than thu Conohifera. A pair of 



muscles called adductors run from the ventral to tho dorsal shell. 

 They are attached to depressions, which are close toge: 

 the ventral sln-11 ; and at their other ends to other depressions, 

 which are wider apart in the donal shell. These in contraction, 

 of com-... .(.-.- tho valves. Arising from the outside and front 

 of these are two i>nir.-d muscles, which are inserted in the dorsal 

 Talve behind the hin^'o-joint, and which thus open tho shell 

 when contracted. Othor muscles go from the ventral and donal 

 to tho stalk, and those, aa well as the pair laiit named, 

 have smaller muscles to assist their action. The position .f 

 these muscles, and of the pits in the aholl to which they are 

 attached, vary in the different speoioa of the class, and form good 

 distinctive characters. Tho 

 calcareous loop also is very 

 different in different spe- ^ * '^^ 

 oies, and sometimes wholly 

 absent. In one whole fa- 

 mily, now extinct, it is 

 twisted into a regular 

 spire of many whorls, 

 which grow smaller and 

 smaller aa they proceed 

 from tho place of attach- 

 ment outward. They are 

 called, from these organs, 

 Spirifers. The arms in 

 these were closely bound 

 to the spires, and little 

 spurs of chalky matter 

 also supported tho ele- 

 ments of tho fringe. Tho 

 Brachiopoda are now rare, 

 especially in the British 

 seas ; but at an earlier ago 

 of the world they swarmed 

 in immense multitudes, 

 then taking the place of 

 the cockles and mussels 

 which we now find. 



The Conchifera are main- 

 ly characterised by their 

 breathing organs. They 

 have no fringed arms 

 stretching away from the 

 sides of their mouth ; and 

 their mantle not being 

 sufficiently effective as an 

 organ of respiration, they 

 have, developed from the 

 body, and lying between 

 the mantle-lined shells and 

 the body mass, two sheets 

 of membrane on each side. 

 The relation of these 

 breathing plates is best 

 seen in tho illustration, whero there is 



is drawn out into two long tabes, on* of wfaiou oocunMuueate* 

 w,th the chamber in which the gilU lie, and the other with Uw 

 mailer lined chamber into which the anas opens, and into 

 also, the gill-tubes disoharge the water. This last 

 arrangement is carried to an extreme in 

 barrow and lire in holes of the rook or mod of the 

 The only communication which these hare with the < 

 is by means of their extended tabes or nphvnt, as they are 

 called. In their case the two tabes are united into one sheath, 

 although a partition passing down the doable tabe always keeps 

 them functionally distinct. In these oies>tures the action of 

 the cilia drives the water in one continuous stream from the 

 gill chamber to the atrial chamber through the tabes; and this 

 motion necessitates a flow down one tabe and up another. By 

 this means floating food is passed along the gills to the month, 

 which is situated at the lower end of the buried molluscs, and 



thus we have a similar 

 mode of life to that of the- 

 tunicate*, though the 

 breathing apparatus is of 

 a different structure. 



The mantle being thi* 

 relieved from the duty of 

 exposing the blood to the- 

 oxygen dissolved in the 

 water, is wholly employed 

 in its proper function of 

 secreting the ahell. It 

 performs this office in a 

 very efficient manner, so as 

 always to allow for the 

 growth of the animal and 

 for the strengthening of 

 the shell as the contained 

 animal becomes more- 

 weighty, and therefore lia- 

 ble to experience more- 

 violent collisions. The- 

 method of secretion is the- 

 following : Bound the 

 edge of the mantle lobes, 

 or at that part where they 

 leave the sheila, are sitn- 

 ated a great number of 

 glands, whence secretions 

 of different substances are- 

 poured out and mingled 

 together. These gland* 

 secrete horny matter, a 



II. 



a section 



the animal as it would appear if it wero cut across 

 to divide both shells. These gill-plates secure that the blood 

 shall bo well aerated, not only by exposing as large an 

 amount of surface aa possible, but also by having gill- 

 tubes, which run through tho plates from one edge to the 

 other, through which the water passes. As is usually the case 

 with breathing surfaces in marine animals, the plates are 

 covered with cilia, whoso motion secures a constant change in 

 the water. Tho gill-plates are very variously modified in the 

 different families of Lamtllibranchiata ; but they are constant 

 throughout tho class. 



In some, as the oyster, the mantle simply lines the shells and 

 ends at their edges, so that the water has free entrance from all 

 sides. In other families the mantle of one side passes across 

 tho aperture of the shell to be united at certain points, or along 

 almost its whole length, to the mautle of tho opposite shell. In 

 others tho animal is not only almost entirely walled in by 

 the union of tho t ,vo lobes of tho mantle, but part of this mantle 



large quantity of carbonate 

 of lime, and some pigment. 

 Thua a fresh rim of hard 

 matter ia added at inter- 

 vals to the shell. The 

 size, shape, markings, and 

 colours of the ahell are all 

 determined by the edges 

 of this mantle; and the 

 whole of these characters 

 given of ] differ so greatly in the different species, and the result is so 



CONCHIFERA. I. CARDIUM. II. DITTO, SIDE VIEW. ITJ. CYTHKRKA. IV. DIA- 

 GRAMMATIC TUANSVEUSK SECTION OF A CONCHIFEB (LAMELUBKANCH). V. 



ISOLATED ALIMENTAKY CANAL OF A SOLEN. 



Kefs, to Nos. in Figs. III. 1, uuibo or elbow ; 2, external ligament ; 3, cardinal 

 teeth of the hinge ; 4, lateral ditto ; 5, anterior adductor ; 6, posterior ditto ; 

 7, mantle lining; 8, the siur.s. IV. 1, shell; 2, mantle; 3, gills; 4, heart 

 traversed by the intestine ; 5, foot inclosing folds of the intestine and vessels. 

 V. 1, mouth ; 2, stomach ; 8, blind sac containing the endostyle ; 4, intes- 

 tine ; 5, heart ; 6, anus ; 7, T, membranous lips. 



beautiful in many, that a collection of shells ia very interesting. 

 The nucleus, or starting-point from which the formation of the 

 shell proceeds, is called the umbo ; and the manner in which the 

 additions are made ia very various. Sometimes the rwtk edge 

 secretes a great deal of matter at one time of the year, and is 

 nearly inactive, or only pours out a thin secretion, at another ; 

 and this will produce a shell with ridges and furrows pamlM to 

 the edges of the shell, which are called mica. If the mantle 

 secretes at certain points, in larger quantity, and but little 

 between these points, or if it be folded or puckered, and the fold* 

 remain so during the whole of the growth, then ridges- and 

 channels are formed, stretching continuously from umbo to> 

 margin. If the margin of the mantle ia much folded and thrust 

 out during secretion, it sometimes results in long points or projec- 

 tions, which reach far beyond the rest of the outside of theshelL 

 In the same way it will be seen that the lining and colouring of 

 the shell into patterns may be effected by the partial and inter- 

 mittent secretion of colouring matter. The shell, while it is being 



