304 SYNOPTIC COLLECTION. 



plains why the posterior region of the body is small and 

 without well developed appendages. 



The adults are often attached to seaweed by one side 

 of the shell (No. 756, S. nautiloides Lam. ; No. 757, S. 

 lucidus} . An examination of this large settlement (No. 

 757) shows a great variation in the tube. If one were 

 unfamiliar with fossil forms, it would seem as if here was a 

 spiral shell in process of making ; as if, in short, a straight 

 tube coiling round the delicate stem of an alga became in 

 time a loose spiral. When, however, one observes that 

 as a rule the young tube at the apex is closely coiled, as 

 we have seen in the specialized fossil species (PI. 753), 

 and that it. is the adult stage which is usually straight, 

 then one sees he is dealing with secondary and not with 

 primitive conditions. The spiral of the young tube in 

 S. lucidus and S. laxus is doubtless inherited from spiral- 

 tubed ancestors, but the tendency to coil loosely around 

 stems is evidently an adaptation to surroundings. This 

 view is strengthened by the fact that those specimens of 

 Spirorbis which are found on the flat fronds of the Fucus 

 (No. 756) are seldom if ever uncoiled to such a degree 

 as those upon stems. 



On a small specimen of Fucus we have seen upwards 

 of three thousand tubes of Spirorbis (2,800 were actually 

 counted, and there were a few hundred more in the hol- 

 lows) not one of which was uncoiled. The whorls of 

 the close spiral of the young tubes could be traced from 

 the inside in many places, where only a portion of the 

 tube had been broken away owing to the strength of the 

 cement by which it was fastened to the Fucus. 



The breathing organs of the living Spirorbis extend 

 from the opening of the tiny coiled tube (No. 756), and 

 are protected when within the tube by the operculum 

 (also seen in No. 756). 



