&ASTEEOPODA. g-jtl 



foot (pro-podium), "which is very distinct in Natica (p. 235), and 

 in Harpa and Oliva ; but is only marked by a groove in. 

 Paludina and DoUum (Fig. 87). The terminal fin (or tail of 

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

 equivalent of the operculigerous lobe [meta-podium) of the ordi- 

 nary gasteropods, such as Sfromhus (Fig. 76). 



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

 part of the body (or cephalo-thorax, M. Edwards) 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 chamber. 

 The heart is proso-hranchiate, although in Firola the auricle is 

 rather above than in front of the ventricle, owing to the small 

 amount of the dorsal flexure. 



The nucleobranchs, and especially those without shells, 

 * ' afford the most complete ocular demonstration of the truth 

 of Milne Edwards's views with regard to the nature of the cir- 

 culation 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 

 backwards 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. Erom 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." (Iluxley.) 



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 branchise ; in 

 Atlanta they are sometimes distinct, and wanting in others ; in 

 Firoloides they are only indicated by a ciliated sub-spiral band. 

 The larvse are furnished with a shell, and with ciliated vela. 

 (Gegenbaur.) 



The nucleobranchs are dicecious ; some individuals (of Firola) 

 have a leaf-like appendage, others a long slender egg-tube 

 depending from the oviduct, and regularly annulated.* The 

 larvae are furnished with a shell and with ciliated vela. (Gegen- 

 baur.) 



The nervous system is remarkable for the wide separation of 



* We can only call to mind one otlier example of a segmented organ in the mcUusca, 

 viz., the penniform styles of Teredo bi]:almulata. 



