ON PHORONIS OVALIS, SI RUT HILL WRIGHT. 
143 
has eighteen handles of longitudinal muscles appears to be 
significant in this connection, though it should be remarked 
that the number of muscle-bundles which I have found in 
P. oval is (cf . PI. 8, figs. 22, 23) appears to exceed eighteen. 
Actinotrocha pallida was described by Schneider 
(1862, p. 64, PI. ii, fig. 12) from Heligoland, where it is said 
to be as common as A. branchiata. It is stated to have 
not more than ten tentacles, which are broader and shorter 
than those of A. branchiata. It possesses only a single 
mass of larval blood-corpuscles, while A. branchiata has a 
pair of these masses, one in connection with each of the 
nephridia. De Selys-Longcliamps (1907, p. 190) has found 
A. pallida at Wimereux (Pas-de-Calais) as well as at Heli- 
goland, and he represents two young stages in PI. xi, figs. 21, 
22. He states that there are never more than six pairs of 
larval tentacles, and that the length of the larva does not 
exceed *6 mm., while that of A. branchiata (p. 189) is as 
much as 2 mm. 
The evidence at present available thus seems to point to 
A. pallida as being the larva of P. ovalis, and the small 
dimensions of this larva are in accordance with the small size 
of the adult form to which it is supposed to belong. 
Summary. 
Phoronis ovalis, which has usually been regarded as the 
immature form of some other species, is shown to be a well- 
characterised adult form. It inhabits burrows which it 
excavates in the shells of molluscs. It possesses in a high 
degree the faculty of regenerating the distal end, which is of 
common occurrence in the genus. Its gregarious habit is 
probably the result of its power of reproducing by transverse 
fission, a process which takes place repeatedly and profusely. 
There is reason to believe that a similar process occurs in 
certain other species which are found as colonies consisting 
of numerous individuals, though it is uncertain whether other 
species have the power of reproducing by fission. 
