THE AMEBIC AN NATURALIST [Vol. XLIV 



belongs to the outer lamella and the other half to the inner lamella of 



In the course of phylogenetic development the originally straight 

 iilaments have become bent upon themselves to permit of greater length 

 (surface) and still be protected within the branchial chamber — those 

 of the ventral series were folded inwards and those of the dorsal (but 

 now lateral) series were folded outwards, while the tips coming in con- 

 tact with the foot in the one case or the mantle in the other clung for 

 support, were directed upwards, and finally became fixed along the side 

 of the body or along the inner surface of the mantle. At places their 

 ciliated surfaces became knit together for further support, leaving 

 intervening gill-slits between contiguous iilaments, and ascending water- 

 tubes between opposite lamellae. At the level of union of the gills 

 with the body and with the manlle there is separated off a branchial 

 chamber below from a supra-branchial (or cloacal) chamber above, and 

 by the activity of the cilia water is brought into the former, directed 



pair of hemibranchs, and is marked by retaining its connection with 

 the body by a septum carrying blood vessels and nerves (Fig. 17). 



Two larval organs soon disappear under the new con- 

 ditions brought about by fixation : the velum and the foot. 

 Even in certain old, free-swimming larvae, that are per- 

 haps belated in their efforts to become attached, a re- 

 duction in the size and vigor of the velum is noticeable. 

 This would seem to suggest an atrophy of the organ, 

 which might be followed by either dehiscence or absorp- 

 tion. I have occasionally seen old larvae with the velum 

 protruded and partly severed from the body, as well as 

 completely separated vela still capable of muscular and 

 ciliary movements. Such cases may have resulted from 

 accidental pinching off by a snapping closure of the 

 shell-valves. But the size, appearance, enfeebled move- 

 ments, and even the accident itself, pointed towards an 

 antecedent loss of ability to respond correlatively to the 

 activity of neighboring organs. 

 Balfour (21, p. 215) states: « It has been suggested by Loven, though 



branchiata are the remains of the velum. The velar area is in any 



Ryder (31, p. 404) says: "The detachment of the ring or crown 



