966 AQUATIC PHYCOMYCETES 



disassociated tube, provided with a single discharge tube, the tip of 

 which at least is ordinarily extramatrical; zoospores fully formed within 

 the sporangium, where movement is initiated, grape-seed-like, with a 

 small refractive anterior granule and two anteriorly attached oppositely 

 directed flagella, swimming individually through a pore formed at the 

 tip of the discharge tube; resting stage not observed. 



The genus, so far as is known, is confined to marine hosts. 



The fungus on which the genus is based is distinct, as Petersen pointed 

 out, from all other related forms in the peculiar septation and sub- 

 sequent disassociation of the tubular thallus. In very early stages the 

 thallus is a somewhat irregular, curved cylindrical structure just within 

 the host wall (Fig. 77 A). x As it matures it elongates and becomes 

 progressively separated by cross walls into a linear series of spherical or 

 cylindrical segments of varying size, each of which becomes an indepen- 

 dent walled unit (Fig. 77 E, G). In many such chains the wall of the 

 original thallus can be detected between the segments (Fig. 77 G). Each 

 of the fragments assumes individual growth, soon loses its original 

 orientation with respect to the "parent" thallus, and ultimately becomes 

 a single sporangium with a discharge tube (Fig. 77 B-D, H). Often 

 when the latter structures are not orientated so as to pierce the algal wall 

 they attain great lengths and bear a striking resemblance to the hypha 

 of a filamentous phycomycete (Fig. 77 F). In this connection it has been 

 observed (Sparrow, 1936b: 254) that fragments of the thallus permitted 

 to develop in close contact with the atmosphere, by allowing sea water 

 to drip over them rather than by submerging them, for example, under- 

 went a remarkable transformation in body structure. Instead of being 

 somewhat olpidioid or moniliform they became extensive isodiametric 

 tubular structures which simulated in every respect, as did the discharge 

 tubes, the hyphae of higher Phycomycetes. Because of these facts it is 

 obvious that Sirolpidium is of very definite interest in the problem of the 

 origin of the mycelium in the biflagellate series of Phycomycetes. The 

 zoospores are grape-seed-like and biflagellate (Fig. 77 I). 



Recent observations by Vishniac (1955b) on Sirolpidium zoophthorum 

 (p. 968), which infects clam and oyster larvae, suggest that the frag- 

 mentation of the thallus may be due to partial anaerobiosis. When she 



1 According to Kobayashi and Ookubo (1953), it is somewhat amoeboid. 



