THE PLANKTON OF ST. ANDREWS BAY _ 7 



SESSIONAL PAPER No. 39b 



Plymouth, England, appeared in the tow at St. Andrew's on August 19th. Whilst 

 it was under examination in a glass vessel, the eversion took place, and the pre- 

 viously free (planktonic) larva was converted into the sedentary (benthonic) 

 worm. 



Almost equally rococo are the free-swimming larvae of the common starfishes 

 at St. Andrew's. Whilst not very abundant, yet they were detected on most days 

 in the plankton between August 10th and 20th. These larvae possess many long, 

 trailing arms. There are in all fourteen arms arranged in two sets of eleven and 

 three respectively. The eleven arms of the first category are simple, elongated, 

 tentacle-like processes, slightly clubbed at their orange-coloured extremities. 

 Along their borders, up one side and down the other, is a narrow refringent zone 

 clothed with vibratile cilia. The ciliated zone or band is continuous at the bases 

 of the tentacles from one to another, excepting that the two tentacles immediately 

 in front of the mouth have their own band continuous with the upper lip of the 

 mouth; while the median anterior tentacle together with the eight posterior ten- 

 tacles have their band continuous with the lower lip of the mouth. Thus there is 

 a pair of pre-oral tentacles with a pre-oral ciliated band; and a series of nine ten- 

 tacles (one median and four pairs) with a post-oral ciliated band skirting them 

 from end to end. It is called post-oral because most of it lies behind the mouth, 

 although as mentioned, it is continued over the median anterior tentacle. 



Occupying the area of the pre-oral lobe in front of the two pre-oral tentacles, 

 there are three arm-like processes crowned with adhesive papillae, and in the middle 

 of the pre-oral lobe, between the bases of the arms, there is a somewhat oval 

 thickening with a few small papillae irregularly scattered around it; this is a median 

 adhesive disc or suctorial plate which can be retracted, i.e., the area which contains 

 it can be pulled back. Of the three adhesive arms, two are ventral, occurring as 

 a pair, and actually arising in the axils of the pre-oral tentacles; the third is median 

 and dorsal. The pre-oral ciliated band is not continued upon the adhesive arms 

 but ceases on each side at the base of the paired arms. This interruption of the 

 pre-oral ciliated band was observed in a young larva which possessed neither 

 arms nor tentacles. ■' 



The cilia are the means of locomotion which consists in an even gliding through 

 the water. The tentacles themselves, although mobile, are not organs of pro- 

 gression, but are sensitive balancers, assisting to suspend the larva in the water. 

 They would represent, therefore, a temporary adaptation to the pelagic habit. 

 When the time of metamorphosis approaches, the tentacles become flaccid and 

 wrinkled, the ciliated rim begins to lose its continuity, and the larva sinks to the 

 bottom where it adheres by means of its adhesive processes and the median 

 sucker. 



Meanwhile the body of the young starfish has been developing in the hinder 

 region of the larva. There is still a certain amount of obscurity surrounding 

 the disappearance of the provisional larval structures and the definite assumption 

 of the starfish form. Soon after the fixation of the larva, the young starfish once 

 more becomes free, but this time as a denizen of the littoral zone of the sea-bottom. 



