COMPARATIVE ANATOMY 



100 



would bo liable to be torn and brained. Add to these incon- 

 vemonouH tho fact that it would be without eyes or fooler* in the 

 fore-part of tint body to direct ita course, or to take observation* 

 of what occurred, and we may judge that the benefit* of travel 

 would bo (|iiito outweighed by its dangers and trouble*. In the 

 gastoro|>ii<K tiirn-fore, both sheila are consolidated into one, and 

 drawn out in an npward direction, HO that, while tho more deli- 

 cate organs are securely lodged, the edge* of the shell's mouth 

 toe withdrawn from tho ground. 



Tho gills are removed out of harm's way in a singular manner. 



IV 



into the gasieropod by placing, side 

 forms between the more typical turbo and the rirer-muMel 

 (Uni.,). In patella (the limpet) it will be sera that the gills are 

 still on both sides of the animal, a* are also the muscles, though 

 these hare no longer the office of cloning the shell, which, in this 

 case, is consolidated into an equilateral eoae. In the bonnet 

 limpet one side of the breathing organs has been aborted, while 

 in turbo both the breathing apparatus and mnsele* of one side 

 are gone, and the whole animal is twisted, in its upper part, 

 into a one-sided spire. In this ease a rounded horny plate if 



I., II., III., IV. DIAGRAMS OF UNIO, PATELLA, CALTPTREA, AND TURBO, TO SHOW THE STAGES OF TRANSITION FEOJI THE COXCHIFEE TO TH rrrtCAi 



GASTEROPOD. V. A SINGLE SET OF TRANSVERSE TEETH FROM (1) A SEPHORATED GASTEROFOD AND (2 AND 3) UNSEPHOEATED GASTEROPODS. VI. 



SHELL OF CASSIS. VII. PALUDINA. VIII. SECTION OF CERITHIUIC. 

 Eef. to Nos. in Figs. I., II., III., IV. 1. 1', lips or tentacles; 2, mantle; 3, shell muscles ; 4, gills; 5, foot; 6, position of the livor; 7, 



byssus; 8, operculum. VI. 1, spire; 2, suture; 3, aperture; 4, outer lip; 5, inner lip; 6, anterior canal for passage of the siphon; 7, 



posterior canal ; 8, varices. VII. 1, umbilicus. VIII. 2, columella. 



Those on one side (usually the right) are brought right up and 

 placed on the animal's back, and there enclosed by a fold of the 

 leathery skin, being placed partially in tho last or largest part 

 of tho shell cavity, while those of, the other side are entirely 

 aborted or dispensed with. This arrangement gives a one- 

 eidedness to the animal, and, perhaps, is the determining cause 

 of the shell being made more compact in the method peculiar to 

 gasteropods, namely, by being twisted into a one-sided spiral or 

 helix, as it is technically called. The head, with ita feelers, 

 eyes, and ears, can be thrust out from the shell and stretched 

 well forward, so aa to gain some acquaintance with those external 

 objects which come within the line of march. In tho illustration 

 it is shown how the conchife? may possibly have been modified 



developed on tho upper part of the foot, or rather tail of the 

 creature, and this, when the animal pulls back its head and 

 thin foot into the shell, closely closes the aperture. This 

 operculum, as it is called, is supposed to be the representative 

 of the horny byssus of the bivalve, being, as will be seen, 

 similarly situated. 



The alimentary canal commences with a month armed with 

 hard parts. These are different in different creatures ; but in 

 all there is a fibrous plate, bearing teeth, placed on a cushion on 

 the floor of the mouth. These teeth are usually directed back- 

 wards ; sometimes the plate in which they are set is very long 

 from point to back, the teeth being disposed in small cross 

 rows set in parallel lines from one end of the plate to the cthec. 



