Mr. J. D. Macdonald on the Anatomy of the genus Atlas. 103 



The generative pit lies at the posterior extremity of the body, 

 and may be retracted or protruded by the action of the longitu- 

 dinal or circular muscular fibres of the integuments. 



A large intcstiniform tube commencing near the inferior part 

 of the base of the proboscis takes a flexuous course backwards in 

 close contact with the abdominal wall, and terminates in an 

 elongated tapering and protrusile organ at the lower part of 

 the generative pit. 



On either side of the oesophagus a convoluted glandular tube, 

 with a ciliated lining, gives rise to a long and narrow duet which 

 passes directly backwards, and ends in a simple orifice lying 

 superior and a little external to the male opening (?) As I have 

 not been able to trace unequivocal spermatozoa or ova in any of 

 these tubes, I am doubtful as to the actual function of the par- 

 ticular organs, but enough has been said to show that Atlas is 

 bisexual. 



The coverings of the body are composed of an external epithe- 

 lium containing purple, brown, or green pigment-granules, and 

 a muscular tunic consisting of an external longitudinal, and an 

 internal circular or subspiral set of fibres. The former are dis- 

 posed in fasciculi with intervals often exceeding their own breadth, 

 but the latter form a continuous layer. 



The interior of the body appears to be lined with vibratile 

 cilia, by the agency of which minute globules may be seen 

 coursing in a definite route through all the open spaces between 

 the viscera. This would appear to be the only representative of 

 a circulatory apparatus ; and that of respiration is most probably 

 combined with it, no heart, distinct blood-vessels or gills having 

 been detected. 



This genus would seem to be made up of gigantic Rotifers, 

 in which the miniature outline, as it were, presented by the mi- 

 croscopic forms is filled up with a more complex internal orga- 

 nization in animals constructed on a much larger scale. It may 

 be also mentioned, that they present characters which give them 

 an intermediate position between the Bryozoa and Tunicata. 

 There is no essential difi'erence between the ciliated circle of 

 Atlas and the circle of tentacula in a polype of Bowerhankia for 

 example. Were the former produced at regular intervals into 

 tentacular processes, Atlas would then only differ from a Bryo- 

 zoon in those particulars which would naturally associate it with 

 the Tunicata. On the other hand, it would appear to represent 

 permanently the larval state of Sipunculus (see Max. Miiller, 

 Mull. Archiv, 1850, v.) ; and in fact it may be regarded as a 

 common centre, connected, as it were, by radiating affinities with 

 a circle of forms differing considerably amongst themselves. 



Cuvier was unable to class the genus, from the ambiguity of 



