198 MANUAL OF THE MOLLUSCA. 



nities entitle them to the first place in the class ; but their extremely aber- 

 rant form, and unusual mode of progression, have caused us to postpone 

 their description till after that of the ordinary and typical gasteropoda. 



There are two families of nucleobranchiate mollusks ; the firolas and 

 carinarias, with large bodies and small or no shells, and the Atlantas, 

 which can retire into their shells and close them with an operculum. Both 

 animal and shell are symmetrical, or nearly so ; the nucleus of the shell is 

 minute and dextrally spiral. 



The nucleobranches swim rapidly by the vigorous movements of their 

 fin-like tails, or by a fan-shaped ventral fin ; and adhere to sea-weed by a 

 small sucker placed on the margin of the latter. Mr. Huxley has shown that 

 these organs represent the three essential parts of the foot in the most highly 

 developed sea-snails. The sucker represents the central part of the foot, or 

 creeping disk (meso-podium] of the snail and whelk; the ventral fin is 

 homologous with the anterior division of the foot, (pro-podium} which is very 

 distinct in Natica (p. 123), and in Harpa and Oliva\ but is only marked by 

 a groove in Paludina and Dollum (fig. 71.) The terminal fin (or tail of 

 Carinaria) which carries the operculum of Atlanta, is the equivalent of the 

 operculigerous lobe (meta-podium] of the ordinary gasteropods, such as 

 Strombus (fig. 69). 



The abdomen, or visceral mass, is small, whilst the anterior part of the 

 body (or cephalo -thorax, M. Edw.) is enormously developed. The proboscis 

 is large and cylindrical, and the tongue armed with recurved spines. The 

 alimentary canal of Firola is bent up at a right angle posteriorly on the 

 dorsal side ; in Atlanta it is recurved, and ends in the branchial cham- 

 ber. The heart is proso-branchiate, although in Firola the auricle is rather 

 above than in front of the ventricle, owing to the small amount of the dorsal 

 flexure. 



The nucleobranches, and especially those without shells, " afford the most 

 complete ocular demonstration of the truth of MILNE EDWARDS' views with 

 regard to the nature of the circulation in the mollusca. Their transparency 

 allows the blood-corpuscles to be seen floating in the general cavity of the 

 body between the viscera and the outer integument and drifting back- 

 wards to the heart ; having reached the wall of the auricle they make their 

 way through its meshes as they best can, sometimes getting entangled 

 therein, if the force of the heart has become feeble. From the auricle they 

 may be followed to the ventricle, and thence to the aorta and pedal artery, 

 through whose open ends they pour into the tissues of the head and fin." 

 (Huxley.} 



Such delicate and transparent creatures would hardly seem to need 

 any special breathing-organ, and in fact it is present or absent in species 

 of the same genus, and even in specimens of the same species. Carinaria 

 has fully-formed branchiae; in Atlanta they are sometimes distinct, and 



