102 Mr. J. D. Macdonald on the Anatomy of the genus Atlas. 



pressure, or elongate themselves so as to assume a vermiform 

 appearance. The specimens which I selected for examination 

 were so continually undergoing those changes of form of which 

 their pliant bodies were susceptible, that it was difficult to find 

 them long enough in one position to portray them with much 

 accuracy. 



The proboscis is supported on a kind of neck, which presents 

 a series of circular creases when partially retracted. The inte- 

 gument round the base of this neck forms an annular spreading 

 fold, bearing on its free border a dense circlet of cilia so large as 

 to be distinctly visible to the naked eye. Although these are 

 the only organs available for swimming, the animals possess the 

 power of rising or sinking in the water at will, without any appa- 

 rent effort. The motion of the cilia is under voluntary control, 

 and the undulations produced by their successive action proceed 

 in a direction from left to right, with a precision and beauty of 

 effect far surpassing those of the ciliated circlets of the Rotifera. 



The species of Atlas creep upon their proboscis, which much 

 resembles both in form and function the foot of a minute Gaste- 

 ropod, but the mouth is situated on the inferior or creeping sur- 

 face. The anterior lip especially expands so as to form a sub- 

 quadrilateral locomotive disk ; but behind the mouth a mode- 

 rately long and bifid lobe projects in a backward direction, the 

 hollow between the two divisions being richly ciliated. 



The upper and fore part or frontal surface of the proboscis 

 meets the creeping disk in front at an angle of about 45°. It is 

 also subquadrilateral in form, presenting a number of rudi- 

 mentary visual organs superiorly, couched in four small patches 

 of black pigment-cells disposed in a transverse row, while on 

 either side it is bounded by a linear elevation, which, together 

 with the superior border, is clothed with large vibratile cilia. 



The oral orifice when open is of a triangular shape, the base 

 corresponding with the posterior lip, but when closed it appears 

 like a simple transverse slit. 



There are no dental organs in Atlas, but the lining membrane 

 of the wide pharynx and oesophagus is thrown into numerous 

 longitudinal folds, tinted with a deep purple pigment. The 

 alimentary canal having formed an elongated gastric dilatation, 

 takes a tortuous course towards the anus, which is situated at 

 the anterior part of the dorsal region, some little distance behind 

 the ciliated circle. 



The liver is massive, minutely lobulated, and lined with secre- 

 ting cells containing globules of a rich golden-yellow oil. The 

 gland is in close relation with the intestine, and the passage of 

 the biliary fluid into the latter is so free, that on the slightest 

 pressure the stomach becomes distended with it. 



