112 



BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



strong basal sphincter lies in the upper lip. This divides ventrally 

 into two branches, an anterior, smaller, which gives rise to a fourth 

 sphincter of the lower lip, and a broad posterior branch which Streiff 

 regards as one of two divisions of the intermediate muscle (his 

 "Bogenmuskel") but it seems rather to be comparable to such a 

 ventral extension of the posterior dorsal lip sphincter as we see in 

 Cyclosalpa. (See figures of both solitary and aggregated forms of 

 Cyclosalpa on plates 1 to 13.) The dorsal horizontal bands are 

 present in the usual position (fig. 104). 



The atrial musculature (fig. 106) is a good deal like that of Iasis 

 zonaria. Both species have a peculiar triangular area of modified 



test, in a valve-like posi- 

 tion on the dorsal side of 

 the atrial siphon. There 

 is a well developed atrial 

 retractor muscle, which 

 is connected at the angles 

 of the atrial aperture with 

 the broad third sphincter 

 of the upper atrial lip 

 and with a ventral branch 

 which soon divides to 

 form the first and third 

 sphincters of the lower 

 atrial lip. There is a 

 strong continuous band 

 of muscle which forms 

 the broad (fourth) sphinc- 

 ter of the dorsal lip and 

 fo . - the second sphincter of 



the ventral lip. The first 



FlG. 105.— THALIA DEMOCRATICA, SOLITARY FORM, ORAL MUSCLES j ^_„„„J -^V. inntnivj nt 



OF THE RIGHT SIDE, VIEWED FROM WITHIN. FROM STREIFF & ™ SCCOnd Sphincters Of 



(1908). the upper lip are more 



delicate. They are united at their base, and distally make something 

 of a network, seeming to correspond to the second sphincter of the 

 upper atrial lip of Iasis zonaria. The first, admarginal, sphincter, 

 which in Iasis is continuous through both atrial lips, is wanting in 

 Thalia. 



The gut forms an elongated loop (fig. 104, B), being bent upon 

 itself in a vertical plane, much as in Traustedtia (pi. 14). The two 

 limbs of the loop are closely appressed in some individuals, but 

 usually are separated by a slight space. The gut protrudes into a 

 short, but well defined, postabdomen which it fills. 



The eye is of the usual horseshoe shape, but shows three slight 

 enlargements, one posteriorly, in the arch of the horseshoe, and one 

 at the tip of each limb of the horseshoe. 



