lOb Mr. W. Clark on the Animals of the Bullidsc. 



i. Bulla hydatis, Linnseus. ;; 



Animal, when fully extended, of an elongated oval shape. The 

 upper or tentacular lobe, and that of the foot, with its reflected 

 portions, are of a pale yellow, aspersed with very minute, close- 

 set, sand-like, dark mouse or lead-coloured points throughout 

 their external surface ; the spots on that part of the mantle co- 

 vered by the shell are larger and of intenser hue, so that the ani- 

 mal has altogether the aspect of being clothed in a close-sprinkled 

 pepper-and-salt jacket. The sinus around the body, which di- 

 vides the upper and lower lobes, is of a pale yellow white colour. 

 The tentacular lobe of the head is small, of subquadrangular 

 form, a little narrowed behind, where, as well as in front, it is 

 gently sinuated, with slight emarginations at the centre of both 

 ends, which by no means give their right and left extremities any 

 pretension to be styled four tentacula, agreeably to M. Cuvier^s 

 view. The true mantle extends to the posterior part of the 

 tentacular lobe, with its thickened rounded margin swelling out 

 a little beyond the periphery of the aperture of the shell ; that 

 part of it covering the branchiae, heart, bladder, matrix and testis is 

 of firm texture ; the portion within the posterior part of the spire, 

 containing the liver, ovarium and oviduct, is a mere film, but 

 stronger than would be supposed from its tenuity ; at the poste- 

 rior end of the shell the dorsal range is completed by the reflexion 

 on it of the hindermost part of the pedal lobe. The anterior part 

 of the under lobe forming the foot is precisely of the same length 

 and width as the upper one, but more posteriorly it spreads like 

 a fin, which is reflected on the sides of the shell ; the disk then 

 pursues its course for two-thirds of its length posteriorly, at which 

 point its continuity is broken by a deep fissure, which only in a 

 marked manner divides the plate, without injuring the fabric ; 

 on the right side near this channel the anus debouches ; the pedal 

 plate is then continued to the posterior end, becoming wider, and 

 there, as before stated, is reflected on the posterior part of the 

 shell. 



It appears then that there are but two lobes, the upper or 

 tentacular, and the pedal one, which though interrupted by the 

 deep groove, is in reality a single plate, but from its posterior 

 and anterior lateral reflexions, together with the tentacular disk, 

 gives the animal the characteristic quadrilobated appearance of 

 the tribe. The object of the pedal fissure is probably to give 

 flexibihty to the foot, and allow the lobes to act as fins, as the 

 animal is equally an adept in natation and reptation. 



Twenty years ago I observed hundreds of these creatures 

 swimming and creeping on the fine mud in the lakes of the 

 Mount Pleasant Warren near Exmouth ; they however suddenly 



