54 COMPARATIVE ANATOMY CHAP. 



Mantle, Visceral Dome. 



Some of the most important points connected with the mantle and visceral dome 

 have already been mentioned (pp. 36-38). 



In Nautilus, the body is attached right and left to the inner surface of the 

 shell of the last or inhabited chamber by powerful muscles, which may make 

 a slight impression on the shell. Between the points of attachment of these 

 lateral muscles, the integument of the visceral dome coalesces with the inner surface 

 of the shell of the inhabited chamber in a narrow circular zone, so that the gas, 1 

 enclosed in the upper chambers of the shell cannot escape. While the integu- 

 ment and mantle beneath this zone of concrescence (i.e. towards the free aperture 

 of the last chamber) are rough, fleshy, and muscular, the integument of that 

 portion of the visceral dome which lies above the zone and is applied to the last 

 septum is delicate and soft. The siphuncle, which arises at the dorsal end of the 

 visceral dome and passes through all the septa, is membraneous and hollow and filled 

 with blood. It is said to communicate with the pericardium. In the female Nau- 

 tilus, the nidamental gland (see Genital Organs, p. 241) lies in the free mantle fold, 

 near the point at which it separates from the visceral dome. "VVe thus have parts 

 which usually lie in the visceral dome wandering into the mantle fold. 



Among the Dibranchia, which are good swimmers, fins are found. In the 

 Octopoda, which are distinguished by the round, compact form of the visceral dome, 

 these are wanting, except in the remarkable genus Cirrhoteuthis. Fins are universal 

 among the Decapoda, and vary much in form, size, and arrangement. 



In Sepia (Fig. 80, p. 83) and Sepioteuthis, the fins are inserted on the lateral 

 edges of the body, along the whole height (length) of the visceral dome, forming the 

 boundary between the anterior and posterior (physiologically the dorsal and ventral) 

 surfaces of the latter. In Eossia, Sepiola, and Sepioloidea they 'are almost 

 semicircular, and are like distinct appendages situated on the anterior surface of 

 the dome, about half-way up it. This is also the case in Cirrhoteuthis, where the 

 more or less circular fin-lobes rise from the body on stalk -like bases. 



The triangular or semicircular fins of Cranchia, Histioteuthis, Onychoteuthis, Loligo 

 (Fig. 34, p. 23), Loligopsis, Ommastrephes, etc., are found at the dorsal end of the 

 visceral dome, on its anterior side. 



In many Dibranchia, there is a concrescence of the free edge of the mantle fold 

 with the integument of the " head" (Kopffuss), which lies below it. This connec- 

 tion is effected by means of a muscular band, which passes over the neck (nuchal 

 band). In most Decapoda, this connection is wanting, and the edge of the mantle 

 is free all round the body ; the exceptions are the genera Sepiola, Cranchia, and 

 Loligopsis, which have a narrow connection of this sort. All Octopoda have this 

 concrescence, commencing with the Argonauta ; it lengthens in Philoncxis and 

 Octopus, till in Cirrhoteuthis it spreads to the posterior surface (physiologically the 

 ventral surface), so that the edge of the mantle remains free only at the aperture 

 through which the funnel or siphon is protruded. 



Arrangements for fastening the mantle fold to the adjacent 

 body wall are very common. Such attachment is either temporary 

 or permanent. In the former case, there are prominences with cor- 

 responding depressions for locking the mantle (appareil de resistance) ; 

 in the latter case, dermal or muscular fusions take place between the 

 mantle and body wall. 



1 Cf. note, p. 37. 



