38 dr. cobbold's description of a 



Description of a new form of Naked- Eyed Medusa (Thaumantias 

 achroa), with brief histological details. By T. Spencer Cob- 

 bold, Esq., M.D. Communicated by the Secretary, 



[Read March 17, 1857.] 



(Abstract.) 



This specimen was obtained from the shore of the Firth of Forth, 

 and presented the following characters: — The form and general 

 aspect of the umbrella resembles that of the more typical species, 

 being hemispherical, transparent, colourless, smooth, slightly elon- 

 gated vertically when in a state of rest, the transverse diameter 

 measuring rather more than the third of an inch and becoming 

 much increased during contraction, the length of the disk at the 

 same time being proportionately lessened. The circumferential 

 portion of the umbrella is fringed by 24 tentacula of extreme 

 delicacy and unusual length ; also by eight ocelli, a circular gastro- 

 vascular canal, and a well-defined shelf-like veil directed inwards. 

 The tentacula, while relaxed and motionless, are fully three times 

 the length of the disk, their particular number and arrangement 

 (5x4-1-4) also constituting a satisfactory mark of identification. 

 Amplified fifty diameters, they exhibit a finely granular and ringed 

 appearance, analogous to that of the prehensile labiate organs of 

 Hydroida ; even with an ordinary pocket-lens indications of 

 knotting may be seen at the extremities of the cirrhi. To the naked 

 eye the tentacular bulbs appear colourless and homogeneous, but 

 under a magnification of 300 diameters linear, the sub-epidermic 

 tissues display numerous closely packed oval or fusiform cells, 

 which refract light very strongly. Near the extremity of the 

 thread, the cells are more cogently developed, and being placed at 

 a right angle to the axis of the filament, appear to stand out from 

 the investing epidermis. At the upper part the tentacula exhibit 

 lateral lines in their interior, denoting the presence of a central 

 canal, the markings becoming more conspicuous near the bidb. 

 Within the bulb the limiting membrane of an otolitic vesicle was 

 discernible, but there were apparently no vibratory movements 

 within the cavity. The ocelli, eight in number (2X4), are placed 

 round the circular margin of the disk, at intervals between every 

 third tentacle — an arrangement somewhat peculiar. Each ocellus 

 consists of a transparent vesicle containing a round nucleus, and 

 in addition five bright yellow, highly refracting globules, the central 

 and superior one being the largest. The sub-umbrella is placed 



