Characteristics of the Mollusca. Ixxxv 



branchial barS;, are collected into a dorsal aorta, which distributes 

 blood to the various organs of the body. A portal system is rudi- 

 mentarily represented by a vessel, which is formed by the veins of 

 the intestine, and sends ramifications to a coecal outgrowth of that 

 tube representing the liver. There is no lymphatic system, nor 

 have any renal organs been discovered in this small Fish. The 

 eye appears to be represented by an azygos pigment speck, sessile 

 upon the anterior prolongation of the nervous axis. No auditory 

 organ has been observed. The generative glands discharge their 

 products by simple dehiscence into the cavity surrounding the 

 branchial sac, whence they escape by the abdominal pore together 

 with the respired water. The ova undergo complete segmentation ; 

 the ciliated embryo is set free before the primitive streak and 

 chorda dorsalis are differentiated, and goes through a peculiar 

 metamorphosis. 



Sub-kingdom, MoUusca. 



Invertebrata, in which the body is bilaterally symmetrical, but 

 often not obviously so ; in which it never is segmented nor pro- 

 vided with articulated appendages ; and in which the length is 

 usually less in relation to the bulk than in either Vertebrata, Ar- 

 thropoda, or Vermes. The organs of animal life often attain but 

 an insignificant degree of evolution in this sub-kingdom; whilst 

 those of vegetative, which are ordinarily massed together in a 

 sacciform envelope, may attain a great predominance in point of 

 size, with which their precedence in order of development is to be 

 correlated. The tegumentary envelope of these latter organs is 

 almost always prolonged so as to form a mantle, which may itself 

 entirely surround the body, and which usually furnishes it with an 

 external shell, or shells, or test. A well-developed digestive tract, 

 consisting of oesophagus, stomach, and intestinal segments, and never 

 opening into the perivisceral cavity, though it is occasionally aproc- 

 tous, is always present in Mollusca. It very rarely takes a direct 

 antero-posterior course, but has almost always its terminal segment 

 bent round so as to be approximated to the mouth, and when it is 



