TYPE MOLLUSCA. 343 



size may be rapidly diminished, remarkable flushes of color 

 passing over the surface of the living animal. 



In the Nautilus (Fig. 160) a chambered calcareous shell is 

 present having a rather complicated structure which will be 

 described later, and in one or two other living forms, such 

 as Argonauta and Spirula, an external shell also exists, but 

 in the majority of forms the edges of the mantle close over 

 the shell, which thus becomes internal and takes the form of a 

 plate lying along the anterior surface of the body, being some- 

 times calcareous as in the common Cuttlefish bone of com- 

 merce obtained from the Sepia (Fig. 152, sh), or else chitiuous 

 as in the common Squid, Loligo. In connection with the 

 mantle there are also frequently developed finlike expansions 

 with a cartilaginous support and provided with muscles, 

 sometimes running along the sides of the visceral hump or 

 in other cases situated near its dorsal extremity. 



The respiratory organs or cteuidia (Fig. 154, ct] are present 

 as either one or two (Nautilus] pairs of pinnate structures 

 lying in the mantle-cavity. Each consists of a central 

 axis attached throughout its entire length to the body-wall, 

 forming a rather high ridge upon it and containing near its 

 outer edge two blood-vessels running throughout its entire 

 length. The vessel nearer the summit of the ridge is the 

 branchial vein carrying the aerated blood back to the body, 

 and between it and the branchial artery is a cavity or canal 

 which communicates with the mantle-cavity between each 

 pair of branchial pinnre. These structures arise from near 

 the free edge of the axial ridge, but each is bound to the ridge 

 by a thin triangular membrane so that they possess the form 

 of lamellse rather than of pinnae. Near the line of attachment 

 of the axial ridge to the body-wall is a cord of cellular tissue 

 richly supplied with blood coming from the branchial artery, 

 forming what is termed a blood-gland, from which the blood 

 is collected into two longitudinal canals which conduct it back 

 to the heart. 



The coeloni of the Cephalopods is characterized by the 

 great development of the pericardial cavity (Fig. 153, pc), 

 which recalls the condition found in the Amphineura, and 

 may perhaps be better termed the viscero-pericardial cavity. 



