102: 



MOLLUSCA 



B-},* in Tan ^eqx'atoBial position. As growth proceeds, one 

 Ihttnisphere FerijaujS 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 Ccelomata. Most remarkable is its 

 agreement with the adult form of the Wheel animalcules 

 or Rotifera, which retain the prae-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 " velum," and is fre- 

 quently drawn out into lobes and processes. As in the 

 Rotifera, it serves the veliger larva as an organ of loco- 



Fio. 8. "Veliger" embryonic form of Mollusea (from Gegenbaur). v, velum; 

 c, visceral dome with dependent mantle-skirt ; p, foot ; (, cephalic tentacles ; 

 op, opcrculum. A. Earlier, and B, later, Veliger of a Gastropod. C. Veli- 

 ger of a Pteropod showing lobe-like processes of the velum and the great 

 paired outgrowths of the foot. 



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 antipodal 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, shs fig. 72***, ss). Thia 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 

 Mollusea, 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 Mollusea 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 are discussed by 

 the writer in the Quart. Jour. Micr. Sci., vol. xvii. p. 

 423. The Veliger, as soon as its shell begins to assume 

 definite shape, is no longer of a form common to various 

 classes of Mollusea, but acquires characters peculiar to its 

 class. At this point, therefore, we shall for the present 

 leave it. 



SYSTEMATIC REVIEW OF THE CLASSES AND OKDEES OF 

 MOLLUSCA. 



We are now in a position to pass systematically in 

 review the various groups of Mollusea, 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 Mollusea 

 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- 

 lusea with their non-Molluscan forefathers having pos- 

 sessed, as do all the more primitive forms of Coslomata, a 

 well-marked prostomium, and consequently a head. The 

 one series of Mollusea 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 GLOSSOPHOEA. Any term 



