Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/667

Rh CLASSES AND ORDERS.] MOLLUSCA 639 B), in an equatorial position. As growth proceeds, one hemisphere remains relatively small, the other elongates and enlarges. Both mouth and anus are placed in the larger area ; the smaller area is the prostomium simply ; the cili ated band is therefore in front of the mouth. The larval form thus produced is known as the Trochosphere. It exactly agrees with the larval form of many Chaetopod worms and other Coelomata. Most remarkable is its agreement with the adult form of the Wheel animalcules or Rotifera, which retain the prse-oral ciliated band as their chief organ of locomotion and prehension throughout life. So far the young Mollusc has not reached a definitely Molluscan stage of development, being only in a condition common to it and other Ccelomata. It now passes to the veliger phase, a definitely Molluscan form, in which the disproportion between the area in front of the ciliated circlet and that behind it is very greatly increased, so that the former is now simply an emarginated region of the head fringed with cilia (fig. 8 ; fig. 6, F ; fig. 7, F ; and fig. 60, A). It is termed the &quot; velum,&quot; and is fre quently drawn out into lobes and processes. As in the Rotifera, it serves the veliger larva as an organ of loco- / Updl^UlUIIl. Ji. E4EU11O1) itllU .U, ItlLCl, CII^CI *J1 a VJrta Ul vf/^^* **&amp;gt; 1 ger of a Pteropod showing lobe-like processes of the velum and the great motion. In a very few Molluscs, but notably in the Com mon Pond-Snail, the emarginated bilobed velum is re tained in full proportions in adult life (fig. 70), having lost its marginal fringe of specially long cilia and its locomotor function. The body of the Veliger is char acterized by the development of the visceral hump on one surface, and by that of the foot on the other. Growth is greater in the vertical dorso-ventral axis than in the longitudinal oro-anal axis ; consequently the foot is rela tively small and projects as a blunt process between mouth and anus, which are not widely distant from one another, whilst the antipedal area projects in the form of a great hump or dome. In the centre of this antipedal area there has appeared (often at a very early period) a gland-like depression or follicle of the integument (fig. 6, C, sk ; fig. 7, E, F, shgl fig. 60, B ; fig. 68, sks fig. 72***, w). This is the primitive shell-sac discovered by Lankester (18) in 1871, and shown by him to precede the development of the perma nent shell in a variety of Molluscan types. The cavity of this small sac becomes filled by a horny substance, and then it very usually disappears, whilst a delicate shell, commenc ing from this spot as a centre, forms and spreads upon the surface of the visceral dome. The embryonic shell-sac or shell-gland represents in a transient form, in the individual development of most Mollusca, that condition of the shell-forming area which we have sketched above in the schematic Mollusc. In very few instances (in Chiton, and probably in Limax), as we shall see below, the primitive shell-sac is retained and enlarged as the permanent shell-forming area. It is sup planted in other Molluscs by a secondary shell-forming area, namely, that afforded by the free surface of the visceral hump, the shell-forming activity of which extends even to the surface of the depending mantle-skirt. Accord ingly, in most Mollusca the primitive shell is represented only by the horny plug of the primitive shell-sac. The permanent shell is a new formation on a new area, and should be distinguished as a secondary shell. The ctenidia, it will be observed, have not yet been mentioned, and they are indeed the last of the charac teristic Molluscan organs to make their appearance. Their possible relation to the prae-oral and post-oral ciliated bands of embryos similar to the Trochosphere will be discussed in the final section of this article dealing with the Polyzoa and Brachiopoda. The Veliger, as soon as its shell begins to assume definite shape, is no longer of a form common to various classes of Mollusca, but acquires characters peculiar to its class. At this point, therefore, we shall for the present leave it. SYSTEMATIC REVIEW OF THE CLASSES AND ORDERS OF MOLLUSCA. We are now in a position to pass systematically in review the various groups of Mollusca, showing in what way they conform to the organization of our schematic Mollusc, and in what special ways they have modified or even suppressed parts present in it, or phases in the repre sentative embryonic history which has just been sketched. It will be found that the foot, the shell, the mantle-skirt, and the ctenidia, undergo the most remarkable changes of form and proportionate development in the various classes changes which are correlated with extreme changes and elaboration in the respective functions of those parts. Division of the Phylum into two Branches. The Mollusca are sharply divided into two great lines of descent or branches, according as the prostomial region is atrophied on the one hand, or largely developed on the other. The probabilities are in favour of any ancestral form the hypothetical archi-Mollusc which connected the Mol lusca with their non-Molluscan forefathers having pos sessed, as do all the more primitive forms of Coelomata, a well-marked prostomium, and consequently a head. The one series of Mollusca descended from the primitive head- bearing Molluscs have acquired an organization in which the Molluscan characteristics have become modified in definite relation to a sessile inactive life. As the most prominent result of the adaptation to such sessile life they exhibit an atrophy of the cephalic region. They form the branch LIPOCEPHALA the mussels, oysters, cockles, and clams. The other series have retained an active, in many cases a highly aggressive, mode of life ; they have, corre spondingly, not only retained a well-developed head, but have developed a special aggressive organ in connexion with the mouth, which, on account of its remarkable nature and the peculiarities of the details of its mechanism, serves to indicate a very close genetic connexion between all such animals as possess it. This remarkable organ is the odon- tophore, consisting of a lingual ribbon, rasp, or radula, with its cushion and muscles. On account of the pos session of this organ this great branch of the Molluscan phylum may be best designated GLOSSOPHORA. Any term