TRANSITION FROM RADIISM TO ANNTJLISM. 251 



the rudeness of their shape, and the general inflexi- 

 bility of their bodies, it is evident that without such a 

 provision they would be, of all animals, the most 

 helpless and incapable. The Sipunculus, on the con- 

 trary (PL V. fig. 1), might, from its external appear- 

 ance, almost be mistaken for an earthworm, or at 

 least for one of those marine Annelidans met with 

 abundantly on every shore. The body of these animals 

 is indeed completely vermiform, although not as yet 

 distinctly annulose, that is, divided into regular rings 

 or segments. In them, to use the words of Professor 

 Forbes, RADIISM sets, and ANNULISM appears. In 

 their internal structure, however, they are distinctly 

 Echinoderms, although even here a transition towards 

 the annulose type is perceptible. They have no rows 

 of suckers ; their bodies are no longer divided into a 

 quinary arrangement of lobes or rays, and the tenta- 

 cula around their mouths are no longer regulated by a 

 definite number. Their motions and habits are those 

 of worms. Instead of having their skins strengthened 

 by calcareous plates or spines, such as have prickly 

 appendages are furnished with horny bristles nearly 

 resembling those seen in many Annelides, whereby 

 their movements are to some extent aided. Poste- 

 riorly the body of the Sipunculus is capable of dilata- 

 tion into a sort of bulb ; while at its anterior extre- 

 mity it is provided with a long fleshy proboscis, at the 

 extremity of which is situated the mouth, surrounded 

 with a flower-like circlet of tentacula. This proboscis, 

 when not in use, is completely retracted into the 

 interior of the animal, folding up like the horns of a 

 snail, until not a trace of it is visible externally. 



