14 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 122 



The variations encountered in the two growth forms of V. argilla 

 are far more fundamental, however, than mere changes in the shape 

 and form of the zoecium. There are extensive changes, for instance, 

 in the morphology of the digestive tract, muscular system, and 

 reproductive system. Many of these changes are related to, if 

 not a direct consequence of, the differences in zoecial shape. The 

 basal-frontal flattening of the stomach in form A is almost certainly 

 a consequence of the flattened shape of the zoecium to which it is 

 confined. The same is probably also true of the differences in arrange- 

 ment of the apertural muscles; the shape of the form A zoecium makes 

 a bilateral arrangement of the muscles around the vestibule a virtual 

 necessity (fig. 6). Other morphological changes are explained less 

 readily. It is difficult to see a relationship, for instance, between the 

 disposition of sphmcter muscles around the cardinal stomach, on the 

 one hand, and the degree of flattening of the stomach or the relative 

 length of the zoecium, on the other. There is clearly a need for more 

 exhaustive research before it will be possible even to propose a reason- 

 able theory of the causes of variability. 



Whereas the closest affinities of V. argilla appear to be with the 

 other members of its genus, there are clear indications of relationships 

 to other genera. Form A of V. argilla is almost indistinguishable 

 from the not uncommon temperate ctenostome Arachnidium Jibrosum 

 Hinks 1880. They are alike in the possession, distribution, and 

 function of filiform processes, the manner of growth (i.e., by the 

 extension, dilation, and anastomosis of tubules), the structure of the 

 apertural muscles, the absence of a gizzard, and the size and shape of 

 the zoecium. In short, a specialist would be hard put to distinguish 

 the two animals were it not for the somewhat denser manner of 

 growth of V. argilla and its gradual extension into the B form. It 

 shoidd be noted, however, th.a,t A. Jibrosum differs from other members 

 of its genus in a number of respects. It possesses filiform processes 

 and the pattern of budding differs somewhat from that of other species 

 oi Arachnidium (Prenant and Bobin, 1956, pp. 223-228). 



Arachnidium Jibrosum (and form A of V. argilla) might be mistaken 

 easily for certain species of the genus Nolella, particularly A^. sawayai 

 Marcus (1938, p. 52), a peculiar species possessing filiform processes 

 but having a long apertural papifla. The close relationship between 

 Nolella and Arachnidium has been noted by Rogick (1949, p. 165), 

 Harmer (1915, p. 58), Ryland (1958, p. 31), and others. Rogick, 

 for example, only hesitantly refers her new species A^. blakei to the 

 genus Nolella, noting (1949, p. 165) that some individuals with very 

 short apertures are better ascribed to Arachnidium. Other zoecia are 

 clearly transitional. 



Nolella, moreover, shows clear relationships to Victorella. Some 



