TYPE MOLLUSCA. 347 



principal venous trunk is the cephalic vein (w), which lies on 

 the posterior side of the oesophagus, and passing dorsally 

 divides into two branches, the venae cavse, with each of which 

 an abdominal vein (oa) unites, the conjoined trunk on each 

 side passing into a contractile dilatation, the branchial heart 

 (bh), at the base of the cteuidium of that side. The venae cavse 

 and the abdominal veins are covered by a much-folded mass 

 of tissue, the venous appendages (tie), which are portions of 

 the nephridia and will be considered in the description of 

 those organs. Mention may also be made here of the peri- 

 cardial glands (pg) attached to the branchial hearts, which 

 have already been described in connection with the viscero- 

 pericardial cavity. 



Slight variations from the arrangement here described 

 may be found in various forms, of which the most important 

 is that found in Nautilus, in which, in accordance with the 

 presence of two pairs of cteuidia, each vena cava divides into 

 two branches, one passing to each ctenidium. No branchial 

 hearts occur, and, as has been already mentioned, the ventri- 

 cle has opening into it two pairs of auricles instead of the 

 single pair usually present. 



In the mesodermal tissue of the Cephalopods in various 

 portions of the body there are developed plates and nodules 

 of a consistency resembling cartilage and like it consisting of 

 a hyaline or partly fibrous matrix through which numerous 

 cells with branching processes are scattered. These cartilagi- 

 nous structures resemble the tissue which is developed in the 

 pharynx of the Gasteropoda below the radula, but reach a 

 much more extensive development in the Cephalopods, serving 

 as a protection for some of the more important organs, and 

 also as a point d'appui for the various muscles, and therefore 

 constituting a true endoskeleton. In the Nautilus there is but 

 a single cartilage which lies on the posterior surface of the 

 ossophagus," being deeply grooved for the reception of the 

 brain and optic ganglia. In other forms, however, the carti- 

 lages are more numerous. There is a well-developed cephalic 

 cartilage forming a deeply-concave disk perforated by the 

 oasophagus, and partially enclosing the brain, being also ex- 

 panded at the sides and hollowed out so as to form a support 



