TYPE MOLLUSC A. 345 



dorsal portion by a transverse fold or partial partition (pa). 

 In Nautilus it is placed in direct communication with the man- 

 tle-cavity by two minute pores, but in other forms such direct 

 communications do not occur. With a ventral prolongation 

 of the ventral cavity the nephridia (N) communicate, and the 

 walls of the cavity fold themselves around the heart (H] in 

 the usual manner, and in addition also enclose the branchial 

 hearts (M), becoming thickened and considerably folded in 

 this region so as to form the so-called appendages of the 

 branchial hearts, which are homologous with the pericardial 

 glands of the Lamellibranchs. The wall of the dorsal cavity 

 is in a similar manner folded over the viscera present in that 

 region, and more or less completely encloses the reproductive 

 organs (ov) so as to form around them a capsule, sometimes 

 with muscular walls, into the cavity of which the reproduc- 

 tive elements are shed when mature. In one group of Ce- 

 phalopods, however, the Octopoda, the arrangement departs 

 slightly from this owing to the reduction of the viscero-peri- 

 cardial cavity to a number of comparatively small canals 

 which constitute the so-called water vascular canals of the 

 older authors. Three of these canals are found on either side 

 of the body, meeting together in a common centre, the ne- 

 phridia communicating with one, another passing to the 

 branchial heart of its side to form the pericardial gland, while 

 the third extends dorsally to dilate with its fellow of the 

 opposite side into the capsule surrounding the reproductive 

 organs (Fig. 158, we). The general relationships of these 

 canals are evidently comparable with those of the viscero- 

 pericardial cavity of the majority of the Cephalopoda, but 

 they differ in one very marked peculiarity, i.e., the heart is 

 not enclosed within their lumen. The tubelike condition of 

 the cavity is evidently a secondary condition, and the exclu- 

 sion of the heart can be understood as a result of the diminu- 

 tion of the extent of the cavity, when the manner in which it 

 is enclosed, as exemplified by the Solenogastres, is considered. 

 The schizoccelic portion of the coalom takes the form partly 

 of lacunar spaces, but partly of blood-vessels with definite 

 walls. To a certain extent the blood system is completely 

 closed, an unusual condition among Mollusca ; well-defined 



