330 MB. A. HANCOCK ON THE ANATOMY 



folds diverge riglit and left, and become continuous with the lower 

 member of what may be termed the anterior cord or collar — two 

 narrow folds also of the lining membrane that encircle the base 

 of the respiratory tube, a little above the anterior margin of the 

 branchial sac, and having the circular blood-channel, previously 

 mentioned, immediately below them. The ventral margin of the sac 

 is furnished with a wide, longitudinal, delicate, membranous fold, 

 which apparently also originates in the lining membrane, and 

 which interrupts the continuity of the minute network in this di- 

 rection. This is the ventral or oral lamina ; it extends from end 

 to end of the branchial sac, and is ribbed transversely ; the mar- 

 gin is entire. The mouth opens close by its left side, about one- 

 third from the lower extremity ; the upper extremity for some 

 little way downwards is divided longitudinally, showing that the 

 lamina is really composed of two lateral membranes ; and each 

 division is united to the lower member of the anterior collar, much 

 in the same manner as the latter is attached to the dorsal folds 

 connected with the endostyle. The upper member of this collar 

 is divided from the lower by a narrow groove, and is uninterrupted 

 by either the oral lamina or the dorsal folds. The oral lamina is 

 connected below by another narrow cord to the posterior extremity 

 of the dorsal folds : this is the posterior cord. 



In this way are traced the boundaries of the two lateral laminae 

 composing the branchial sac. They are attached by their upper 

 borders to the walls of the pallial or respiratory chamber, a little 

 below the anterior collar or cord ; the dorsal margins are attached 

 along the sides of the endostyle, and the lower margins along 

 the line of the posterior cord. In all other parts the two lobes 

 are free, except at the points where the suspenders bind them to 

 the walls of the chamber, and where the extremity of the oesopha- 

 gus penetrates the branchial sac ; and here, of course, the latter 

 is attached to the alimentary tube. The supposed function of the 

 endostyle has been already indicated ; the folds of the lining 

 membrane to which it is adherent are no part of the gill ; neither 

 can the oral lamina be considered a portion of the breathing- 

 organ: it is certainly highly vascular; that is, minutely ramifying 

 blood-channels can be traced in it ; but similar vessels (or chan- 

 nels) are seen in all the membranes of the organism, and also occur 

 in the dorsal folds in connexion with the endostyle. The office of 

 the oral lamina is to conduct the food to the mouth. And it has 

 already been stated that the sedimentary matters are there accu- 

 mulated and formed into a cord, and so carried to the oral aper- 



