REGULATION OF HARENACTIS ATTENUATA. 3 



length of the animals was never much greater than the depth 

 of the layer of sand. On the other hand, when they live in a 

 sufficiently thick layer of sand they may attain a length of 40-50 

 cm. The greatest diameter of the column near the oral end is 



o 



15-25 mm. From this region the column gradually tapers to 

 an extremely attenuated proximal portion, which differs greatly 

 in length at different times and in different individuals. 



When the animals are removed from their burrows the aboral 

 end is usually more or less button-shaped or disc-shaped (Figs. 

 1-4) and firmly attached to a bit of shell, a small stone or other 

 solid object. It is, in short, structurally and functionally a foot. 

 In removing the animals from the sand this foot is often lost, 

 since the attenuated region just distal to it is relatively weak. 

 Sometimes, if the foot happens to be attached to some object 

 near the surface -- within 510 cm. the attenuated region may 

 be completely absent and the animal may possess something the 

 shape of Fig. 10. 



In some cases, however, the foot is not attached to any solid 

 object ; under these conditions it may take the forms shown in 

 Figs. 5 and 6 or almost any form intermediate between them. 

 No well defined adhesive surface is developed in these cases, 

 though grains of sand may in some cases adhere to some part of 

 the region. These marked differences, occurring under natural 

 conditions, suggest that the differentiation of a localized adhesive 

 foot is dependent, at least in some degree, upon contact with a 

 solid substratum, a suggestion which is confirmed by the facts 

 cited below. Such forms of the aboral end as those in Figs. 5 

 and 6 undoubtedly result from the failure of this region to come 

 into contact with a large solid body. 



The diameter of the disc is scarcely greater than the greatest 

 diameter of the column ; the tentacles of large individuals, when 

 fully extended may reach a length of 40 mm. 



The circular muscles of the body-wall are strongly developed 

 throughout and contraction is often unequal in different parts of 

 the body, so that marked constrictions occur. The mesenteries 

 bear powerful longitudinal retractor muscles, which are inserted 

 distally near the free end of the oesophagus. Contraction of 

 these muscles invaginates the disc and tentacles (Fig. 7) and in 



