Mr. J. 1). Macdonald on the Anatomy of the genus Atlas. 101 



in the number of tentacula, &c. is of little consequence, for I 

 have seen, and have a long list of notes and numerous drawings 

 of the strange changes, from the young to the adult state, of 

 these lovely gems ; at present I cannot spare the time to make 

 the drawings and extend the notes. 



Custom House, Wick, N. B., 9th Nov. 1855. 



EXPLANATION OF PLATE VIII. ^^ 



All very highly magnified. 



A. A group of the animals on a piece of shell. 



A, a. One with slender additional tips to the tentacles. 



B. One of the animals more highly magnified. 

 Fig. 1. One of the Medusoids. 



Fig. 2. Ditto when changing. 



Fig. 2 a. Stomach and lips. 2 b. Upper part of umbrella. 



Fig. 3. Under side, showing the arrangement of the tentacula, &c. 



Fig. 4. Edge of mantle to show the canal, &c. 



Fig, 5. One of the disks at the end of a tentacle. 



Fig. 6. The stomach, mouth and lips. 



Fig. 8. Upper part of the umbrella, showing the arrangement of the gastro- 



vascular canals. 

 Fig. 9. A tentacle to show the markings. 

 Figs. 10 & 11. The Medusoid changed and turned inside out. 



XII. — Brief Outline of the Anatomy of the genus Atlas [Lesueur), 

 By John Denis Macdonald, Assistant-Surgeon of H.M.S.V. 

 " Torch,'^ Tender to H.M.S. " Herald,^' Capt. Denham, R.N., 

 F.R.S., Commanding the Exploring Expedition in the South 

 Seas. 



[With a Plate.] 



During our late cruises between Sydney and the islands of the 

 Pacific, different species of Lesueur^s curious genus Atlas were 

 taken in the towing-net, and the following short anatomical 

 account of them may prove interesting to the zoologist^ more 

 especially as — like Phyllirrhoe — their position in the animal 

 kingdom has been so much a matter of doubt. 



These little animals are of a rounded, oval, or elongated form, 

 according to the amount of contraction of the longitudinal and 

 circular muscular fasciculi ; and they are usually between ^th 

 and -(jth of an inch in length. 



Many of their movements resemble those of Annelida, parti- 

 cularly the manner in which they protrude and retract the head 

 and proboscis. They frequently draw up their bodies into the 

 form of a sphere, enabling them to resist a very considerable 



