in DOLIOLUM LIFE-HISTORY 99 



oozooid, and are sacrificed for the benefit of the rest of the 

 colony. They serve merely to aid in respiration, and to provide 

 the food for the nurse and the median buds. Their development 

 is arrested ; they have the body elongated dorso-ventrally with a 

 large funnel - like branchial aperture (Fig. 60, F), and the 

 musculature is very slightly developed. 



(2) Some of the median buds become foster forms (" phoro- 

 zooids "), which, like the preceding trophozooids, do not become 

 sexually mature, but, unlike them, are eventually set free as 

 cask-shaped bodies having the Doliolum appearance, with eight 

 encircling muscle-bands, and having, moreover, a ventral out- 

 growth (not a stolon), which is formed of the stalk by which 

 the body was formerly attached to the dorsal process of the 

 oozooid. On this ventral outgrowth the " gonozooids " (3) are 

 attached while still very young buds, and after the phorozooids 

 are set free these reproductive forms gradually attain their com- 

 plete development, become sexually mature, and are eventually 

 separated off, finally losing all trace of their temporary connexion 

 with the foster-forms. They resemble the foster-forms in having 

 a cask-shaped body with eight muscle-bands, but differ in the 

 absence of a ventral process, and in having the sexual repro- 

 ductive organs fully developed. 



Occurrence. The best -known member of the genus is 

 Doliolum tritonis, Herdman, which was captured in the tow-nets 

 in thousands by Sir John Murray during the cruise of H.M.S. 

 " Triton" in the summer of 1882 in the North Atlantic. Since 

 then that species, or the closely allied D. nationalis, Borgert, have 

 been found on more than one occasion in the English Channel 

 and other parts of our south-west coasty and so Doliolum may 

 be regarded as an occasional member of the British surface 

 fauna. 



It is probable that the occasional phenomenal swarms of 

 Doliolum which have been met with in summer in the North 

 Atlantic are a result of the curious life-history which, under 

 favourable circumstances, allows of a small number of oozooids 

 producing from minute buds an enormous number of phorozooids 

 and gonozooids. 



As the result of the careful quantitative work of the German 

 " Plankton " expedition, Borgert thinks that the temperature of 

 the water has more to do with both the horizontal and the 



