122 



DISCOVERY REPORTS 



as an oblique continuation of the long axis of the body, as in P. legumen. If the ratio of the height 

 (i.e. dorso-ventral diameter) of the body to the length of the stalk is plotted against the total length of 

 the specimen, it is seen that this ratio decreases with increase of total length, that is presumably with 

 age (Text-fig. 49). Old animals may have a stalk from three to eight times the dorso-ventral diameter 

 of the body, whereas in young animals the stalk may barely equal the diameter of the body. As 

 in P. legumen the surface of the body bears a sparse but variable coating of spines (Text-fig. 47 E). In 

 P. georgiana however, the spines usually lack the secondary thorn-like processes commonly found in 

 P. legumen. 



Parietal organs. This species has no atrial organs within the base of the atrial siphon, but in every 

 specimen dissected a parietal organ was found on the middle of each side of the body. Sections show 



7-1 



6- 



5- 



Hl-J 

 ca\<n 



O 

 I 



3- 





I 1 1 1 1 1- 



10 15 20 25 30 35 



TOTAL LENGTH CM 



40 



-1 

 45 



Text-fig. 49. Pyura georgiana Michaelsen. Relation between the ratio body-length/stalk-length and the total length. 



that the parietal organs consist of a number of closely packed rounded vesicles (Text-fig. 50 A, v.), 

 each vesicle being composed of many vacuolated cells (Text-fig. 50 B, v.c). It is not clear whether 

 these cells remain separate although crowded together, or whether they form a syncytium. In fixed 

 and sectioned material the vacuoles of the cells are empty, and it is not possible to say whether the 

 parietal organs may be stores of reserve material, or, as Kott (1954) suggests, renal organs. 



Arnback (1938) has well described the remaining internal organs of the species. 



Biology. One large animal from St. 123, collected on 15 December 1926, has several small 

 specimens of the same species attached to its test. These young ones had a body ranging from 0-25 

 to 0-70 cm. in diameter and apparently represented a settlement from one breeding season. Without 

 knowing the rate of growth, however, it is not possible to say when breeding took place but it may have 

 been within the previous month, that is in late November or early December. 



Remarks. The range of depth given by van Name (1945) suggests that this is a shallow water species, 

 but in fact it has generally been taken from depths of 100-200 m., although Michaelsen's (1900) 

 original record was from water of 16-25 m - Most of the 'Discovery' material is from depths of more 

 than 100 m. 





