vii MOLLUSC A— RESPIRATORY ORGANS 97 



of it streams through the "blood-making " gland, and returns to the venous branchial 

 heart still impurified. There are, further, certain fine branchings of the branchial 

 artery which serve for nourishing the gill and its suspending membranes. The blood 

 in these returns to the venous sinus through a special vessel which runs parallel to 

 the branchial artery on its anterior side. 



A powerful nerve enters the gill at its base and ramifies through it. A muscle 

 spreads over the surface of the "blood-making" gland, and a special musculature 

 brings about the contractions of the principal branchial vein. 



The gills of the Odopoda differ considerably, though not essentially, in structure 

 from those of the Decapoda. The branchial channel is much larger, and the leaflets 

 are not only folded, but have on each side alternating lamellae, which in their turn 

 may carry similar lamelUu of the second order, and so on till in some cases the 

 seventh order of subsidiary lamellse is reached. The leaflet is thus an extremely 

 complicated, folded, or feathered structure with its surface increased to an 

 extraordinary degree. 



Adaptive Gills. 



The Scaphopoda and many Gastropoda possess no true ctenidia. 

 In the Fulmonafa and the few air-breathing Frosobrcmchia, the ctenidia, 

 as organs adapted for aquatic respiration, have disappeared. It is, 

 however, at present difficult to determine the cause of their dis- 

 appearance in Opisthohranchia which inhabit water, and in the gill-less 

 forms of the Pteropoda, all the more so, as in most Opisthohranchia 

 they are replaced by adaptive gills, which are new structures in no 

 way comparable morphologically with ctenidia. These adaptive gills 

 may even appear (Pneumoderma) before the true ctenidia have dis- 

 appeared. The Scaphopoda and many Opisthohranchia have no gills 

 whatever, and in these respiration evidently takes place at various 

 suitalile parts of the surface of the body. In many cases, also, where 

 epipodial or parapodial processes are developed as well as gills, or 

 the mantle possesses extensions, these may help the gills in the 

 function of respiration. 



Adaptive gills are found in most Ascoglossa and in the Nudi- 

 branchia ; also, as mentioned above, in the gymnosonuitous I'teropoda. 

 In the latter, they consist of small fringed or plain ridges at the 

 posterior end of the body ; these may be of various shapes ; a 

 description of them would be of no special interest to the comparative 

 anatomist. 



The principal forms of adaptive gills of the Nudibranchia are : 

 (1) the anal gills of the Dorididm ; (2) the longitudinal rows of 

 branchial leaflets to the right and left under the mantle fold of the 

 so-called Phi/Uidiida^ ; (3) the dorsal appendages or eerata of the 

 Nxtdibranchia and most Ascoglossa. 



1. The Anal Gills (Fig. 93). — These take the form of delicate leaflets, generally 

 feathered on lioth sides, which, in the Dorididoe, form a rosette round the anus, 

 which has a median dorsal position towards the posterior half of the body. Cerata 

 may occur with the anal gills (Pobjceridce). The view that these gills are ctenidia 

 has as yet no sufficient foundation. 



VOL. II H 



