EEPOET ON THE PTEEOPODA. 
83 
On dissecting an Aplysia and a Pneumonoderma it will be seen that the fins of the 
latter and the parapodia of the former are innervated in exactly the same manner, similar 
nerves pass to them, issuing from the same points in the pedal ganglia. 
In the Bulloidea, the parapodia (specially developed in Gastropteron, Acera, &c.) are 
continuous with the plantar or creeping surface of the foot, and form with it an unin- 
terrupted surface. In the Aplysioidea the more ventral portion of the parapodia serves 
as a part of the visceral wall (Fig. 3, B), so that their origin appears to be separate from 
the plantar surface ; a similar arrangement is found in the Gymnosomatous Pteropods, 
where, in consequence of the reduction of the plantar surface, the parapodia or fins seem 
still further separated from the latter. 
Fig. 3. — Diagrammatic transverse sections, A, of one of the Bulloidea, B, of one of the Aplysioidea ; 
a, creeping surface of the foot ; b, parapodium or natatory lobe of the foot. 
e. 
Among the Aplysioidea are found different degrees of freedom of the parapodia 
relatively to the visceral sac, which lead gradually to the Gymnosomatous type. Thus 
in Aplysia leporina the parapodia are largely united behind ; in Aplysia punctata they 
are less so ; in Aplysia fasciata they are for the most part free. 
In all these, however, the plantar surface is fused with the visceral mass, to the 
posterior extremity of which it extends, and 
the parapodia reach to the same point as the 
plantar surface. In Notarchus, on the other 
hand (Fig. 4), the plantar surface has no con- 
nection with the visceral sac, and the two 
parapodia are united dorsally above this latter, 
being fused throughout their whole length 
except a small tract anteriorly ; they form 
thus a sac in which floats the visceral mass. 
CL 
In the same manner in the Gymnosomata the Fig. 4.— Sagittal section of Notarchus J a, creeping surface of 
, the foot ; b, parapodia united so as to form a sac around 
foot has no connection with the visceral sac ; the visceral mass ; c, aperture of the sac ; d, head with 
i n i -i anterior and posterior tentacles. 
but here the plantar surface being reduced to 
the anterior part of the body, the parapodia or fins are also reduced to the same 
portion. 
If, however, the fins of the Gymnosomata are homologous with the parapodia of the 
