. Elements of Comparative Anatomy. EXCEETOEY OEGANS OF MOLLUSCA. 379 § 292. The great variations in the more special characters of the excre- tory organ, which are seen in the Gastropoda, prevent our being astonished at the fact that this same organ is otherwise modified in the Cephalopoda. All the Cephalopoda possess in their visceral mass closed sacs, which open into the mantle-cavity. As the efferent ducts for the generative products resemble the excretoiy canals in having that part of them which surrounds the germ - glands connected with the coelom, it is pro- bable that these efferent dac
. Elements of Comparative Anatomy. EXCEETOEY OEGANS OF MOLLUSCA. 379 § 292. The great variations in the more special characters of the excre- tory organ, which are seen in the Gastropoda, prevent our being astonished at the fact that this same organ is otherwise modified in the Cephalopoda. All the Cephalopoda possess in their visceral mass closed sacs, which open into the mantle-cavity. As the efferent ducts for the generative products resemble the excretoiy canals in having that part of them which surrounds the germ - glands connected with the coelom, it is pro- bable that these efferent dacts have been derived from primitive excretory organs; in this case the Cephalopoda must have had a larger number of these organs, of which some alone have retained their primitive significance. Four of these organs are found in Nautilus, and two in the Dibranchiata. Their orifice is sometimes placed on a papilliform process (Fig. 178, r). The large branchial vascular trunks project into these sacs, and their walls, therefore, are irregular in form. So far, however, as these vessels project into the sacs, their walls must be regarded as belonging: to the sac. In connection with the bran- chial arteries the wall of each sac presents numerous ramified appendages, which project into the lumen of the sacs (cf. Fig. 178, li; Fig. 202, re); they are formed by ctecal diverticula of the vessel, and a superjacent glandular investment. In Nautilus these appendages of the four venous trunks are clothed by tubular glands which open into the connected sac. It is still doubtful what we should regard the appendages on the other blood vessels, which project into the pericardial sinus, as being. As this sinus communi- cates with the mantle-cavity, perhaps they also represent excretory organs. In the Dibranchiata the venous appendages appear to have a somewhat different structure. Concretions of phosphate of calcium appear to be the principal products of this appa- ratus ; in the Sepiadae this o
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