340 INVERTEBRATE MORPHOLOGY. 



shell-valves to close with considerable force and the teeth to adhere to any 

 soft object they come in contact with. By these arrangements the Glo- 

 chidia are able to fasten themselves to the skin of the fins or to the gills of 

 fishes, where, setting up an inflammation, they become enclosed in a cyst 

 within which the organs assume the adult form, the embryo assuming a 

 truly parasitic habit and drawing nourishment from the tissues of its host. 

 When sufficiently developed the young mussel makes its way out of the 

 cyst and assumes a free mode of life. 



As regards the affinities of the Pelecypoda there can be no doubt that 

 between the Protobranchiate forms and the Diotocardiate Gasteropoda 

 there are not a few resemblances. They possess a creeping foot, true 

 ctenidia function as gills, the heart is traversed by the rectum, the pleural 

 and cerebral ganglia are distinct, and the nephridia serve to transmit the 

 reproductive elements to the exterior, all of which are also to be found in 

 certain of the Diotocardiates, such as Haliotis and Fissnrella. It may be 

 supposed that the Pelecypods arose from the Gasteropod stem before the 

 asymmetry became developed, and subsequently, by assuming a fixed or 

 limicolous mode of life, a certain amount of degeneration, such as the loss 

 of the radula and of tentacles and cephalic eyes, supervened. The plough- 

 share-shaped foot is undoubtedly an adaptation to the limicolous habit, 

 and the great development of the gills stands in relation to their mode of 

 obtaining food, the cilia of the gills being responsible for the production of 

 currents sufficiently strong to carry with them diatoms and other small 

 organisms upon which the Pelecypods feed. 



V. CLASS CEPHALOPODA. 



The Cephalopoda are in some respects the most specialized 

 of all the Mollusca, but nevertheless present the primitive 

 bilateral symmetry and arrangement of the structures associ- 

 ated with the mantle-cavity. The visceral hump is enor- 

 mously developed (Fig. 152), so that the true anterior and 

 posterior surfaces of the body are very long, whereas the 

 ventral surface is comparatively short, the general form of 

 the body being not unlike that of the Scaphopoda. Unlike 

 the members of this latter class the Cephalopods lead, how- 

 ever, a somewhat active existence, some, such as the Squids, 

 swimming actively about in the sea to which they are exclu- 

 sively confined, while others, such as the Cuttlefishes, have a 

 more creeping habit, though capable of swimming freely. 

 While swimming the animals assume a position in which their 

 longest axis, i.e. the dorse-ventral axis, is more or less hori- 

 zontal, the true morphological anterior surface thus becoming 



