284 BULLIDJE. 



draws up the shell to meet the extension, its edges catch and 

 raise them to an erect posture. 



The eyes, though not large, are distinct and fixed, in those 

 species in which they are not obsolete, as in B. hydatis, and I 

 believe in all other Bullae, at the anterior internal bases of the 

 so-called tentacular processes; they are not raised, but im- 

 mersed in the surface of the skin : though they and the flaps 

 or fins are in rather an advanced position, they are only seen 

 when the animal is on the march, from its habitude at other 

 times of keeping these organs under the shell ; but in those 

 of thin texture they may be seen through it. The foot, as in 

 B. cylindracea, is one entire lobe, divided as in the type at 

 the under part from its posterior linear accessory, which also 

 lies within the mantle in the narrow aperture of the shell, by 

 a deep solution of continuity. In this minute creature it is 

 difficult to observe the connection and origin of the accessorial 

 lobe with its principal : the deep groove or rather hiatus be- 

 tween the two portions of the foot causes the anterior division 

 to assume the appearance, as M. Loven terms it, of " solea 

 brevissima;" the mistake is very natural; the free floating 

 margins resulting from its apparent division from the posterior 

 lobe, give the appearance of a complete foot. The anterior 

 part of the foot is rather shorter than the head-lobe, ovately 

 subquadrangular and rounded at the front and posterior ex- 

 tremities, sinuated at the centre, grooved around the margin, 

 and not at all auricled. I have observed thirteen of these 

 animals for four or five days in sea- water, but I never saw 

 any reflexion of the mantle ; however, the foot at times is 

 considerably reflected laterally on itself and the anterior part 

 of the shell, and in quietude assumes the quadrilobated 

 character of the tribe. The march of. this animal is much 

 more lively than in B. cylindracea. The mantle forms an 

 anal sinus at the posterior part of the canal of the shell ; there 

 is a deep groove between the head and foot-discs. The bran- 

 chial plume is deposited in a cavity across the neck. The 

 buccal mass is a light fleshy palate, and may be seen through 

 the head-lobe ; the mouth leads by a long linear oesophagus 

 to the stomach, situate in the last convolute cavity of the 



