498 LECTURE XX. 



ation the clijle or product of digestion must transude through the 

 intestinal tunics into the intestinal sinus*: all the fluid answering to 

 lymph, from other parts of the organisation, will be received into the 

 intervisceral, intermuscular, and pallial sinuses, which will perform 

 the double but closely allied functions of veins and absorbents. 



The streams of sea-water excited and maintained by the complex 

 ciliated structure of the mantle f, will effect the requisite respiratory 

 changes as they course over the delicately coated, branched vessels 

 of the pallial lobes ; these, therefore, form the chief seat of the 

 breathing function ; but, wherever similar currents come in contact 

 with the vascular system, to that extent the respiratory operations 

 will be diffused. In CCCIII. I suggested that the minute caecal pro- 

 cesses continued from the mantle into the shell-tubes " might perform 

 an excretory function and be associated in that action with the de- 

 purative respiratory office of the mantle, the probable condition of 

 their development being the low grade of the proper branchial organi- 

 sation." (p. 4.) Dr. Carpenter, believing that certain corpuscles which 

 he has observed in the caecal processes are blood-corpuscles, deems it 

 not improbable " that the apparatus in question is branchial in its 

 nature." J 



The generative organs present the same form in the Terebratulce^ 

 OrhiculcB, and Lingulce, and correspond with that of the pallial 

 sinuses in which they seem to be situated. The figures 15 & 16 r, r, 

 pi. 22, CCCL, and fg. 2, pi. 2, CCCIIL, represent the general 

 form and disposition of these organs in Terebratula, I have 

 observed, however, a manifest difference of texture and colour in 

 the generative organs of different individuals of Ter. fiavescens 

 collected at the same period. In some specimens the organs are 

 better defined, more compact, and of a paler colour : in others 

 the organs are broader or more diffused, and of a deeper yellow 

 colour. On a microscopic examination of the first kind, they 

 are found to consist of an aggregate of minute cells, closely 

 resembling those in the half mature testes of the common oyster. 

 I conclude, therefore, that they are the male generative organ in the 

 Terebratul(Bi and that the individuals of this genus are dioecious, 

 not, as I formerly supposed, simple hermaphrodites. § The true ova are 

 very plainly manifested in the broader and deeper-coloured dendritic 

 organs. They are developed, like the sperm -cells, between the mantle 

 and the delicate tunic of the venous sinus, apparently form the accom- 

 panying branched organ, and protrude into the sinus, pushing its 



* CCCIir. pi. 1. 2. 8. t CCCI. pi. 23. fig. 13. CCCVIII. tab. 1. fig. 13. 

 X CCCX. p. 36. § CCCI. 



