368 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 2 



SPECIFIC DIAGNOSIS OF OPEGASTER ACUTA 



Body elongate, little flattened, almost cylindrical, widest at acetabular 

 level, tapering toward each end, usually rather sharply pointed at pos- 

 terior end; 0.950 to 1.633 in length, 0.307 to 0.427 in width. Forebody 

 0.187 to 0.382 or about % body length. Oral sucker usually longer than 

 wide, 0.120 to 0.150 in width by 0.135 to 0.150 in length. Acetabulum 

 wider than long, with transverse aperture; nonpedunculated ; 0.210 to 

 0.232 in diameter; sucker ratio approximately 3:5. Acetabulum with 5 

 pairs of small, simple, interlocking papillae. Prepharynx short and wide ; 

 pharynx 0.085 to 0.110 long by 0.060 to 0.102 wide; esophagus short and 

 wide; intestinal bifurcation about midway between suckers; ceca unite 

 near posterior end to form a short rectum (of somewhat variable length 

 up to 0.102) ; anus ventral, subterminal, close to excretory pore. Genital 

 pore median, opposite posterior edge of oral sucker. Testes subglobular 

 (may be wider than long or longer than wide, the posterior testis being 

 usually more elongate), tandem, close together, immediately posterior to 

 midbody, intercecal, smooth. Cirrus sac very small, inconspicuous, sur- 

 rounding only the cirrus, not extending posterior to pharynx. Seminal ves- 

 icle a sinuous tube lying free in parenchyma from level of intestinal bi- 

 furcation to about middle of acetabulum, connected with the cirrus sac by 

 a thin-walled uncoiled tube. Prostate gland apparently lacking but per- 

 haps represented by large nuclei in region of seminal vesicle. Ovary me- 

 dian, immediately pretesticular, either subtriangular and unlobed or dis- 

 tinctly trilobed; yolk reservoir to left of ovary; seminal receptacle lack- 

 ing, sperm cells in uterus; Laurer's canal present; uterus preovarian; 

 eggs yellow, thin shelled, blunter at one end, 44 to 49 by 24 to 27 {x (in 

 balsam mounts). Vitelline follicles filling sides of hindbody; confluent 

 posterior to testes ; dorsal, ventral, and lateral to ceca ; may overlap tes- 

 tes dorsally; extending anterior to acetabulum almost to intestinal bifur- 

 cation on one or both sides. In one specimen (fig. 41) only rudimentary 

 follicles could be seen anterior to acetabulum. Excretoiy vesicle extending 

 forward to ovary. 



Comparisons. Opegaster acuta differs from all other species in the ge- 

 nus in the location of the genital pore, which is much more anterior. Most 

 species do not have as many as 5 pairs of acetabular papillae. O. acuta dif- 

 fers from O. tamori in extent of the seminal vesicle, in shape of testes, in 

 egg size, and in length of esophagus. 



