242 THE STRUCTURE AND AFFINITIES OF HERDMANIA CLAVIFORMIS. 



branchial siphon is considerably larger than the atrial, and is bent over toward the 

 ventral or endostylar side, so that the plane of the orifice usually forms almost a right 

 angle with the plane of the section of the siphon at its base. The orifice is six-lobed, 

 the dorsal lobe being much larger than any of the others. An average condition of the 

 lobes and orifice when the latter is nearly closed is shown in front view in Figure 4. 

 The atrial siphon is considerably smaller than the branchial; it is nearly straight, 

 though inclined somewhat away from the branchial, and is also six-lobed, the lobes 

 being in general of about equal size. 



Branchial tentacles are numerous. There are thirty or forty or more that are well 

 developed, and in addition a variable number of small ones. The arrangement is 

 peculiar. There is a bunch of about ten or twelve immediately in front of the hypophy- 

 sis; on each side of this bunch comes a considerable area entirely devoid of tentacles; 

 then, following these bare areas, come the majority of the tentacles uniformly dis- 

 tributed over the remaining portion somewhat more than one-half of the circle. 

 The individual tentacles are simple, rather long and slender, with enlarged bases, 

 and nearly round in cross-section. The outermost tentacles are close to the peri- 

 pharyngeal band. Figure 5 illustrates these several points, although a portion of 

 the tentacular field of the specimen from which this figure was taken was broken 

 away. 



No atrial tentacles are present. 



The peripharyngeal band is well developed. The area between it and the first 

 series of branchial stigmata is small. 



The branchial sac is without folds, internal longitudinal vessels, or papillae. 

 There are usually twelve series of stigmata, but the number may vary in either 

 direction by one or two series. Each series has about forty stigmata of quite regular 

 form, size, and arrangement. 



The transverse vessels carry membranes that project prominently into the interior 

 of the branchial sac. There is a single, moderately developed dorsal languet for each 

 of these vessels (Fig. 6). 



The two rapheal muscle-bands are well developed and extend the entire length of 

 the sac (PL XVIII, Fig. 5; PL XIX, Fig. 15, mu. rph.). They are as usual situated 

 close along the dorsal blood-sinus. There are also two or more muscle-bands running 

 lengthwise of the animal ventral to the endostyle, that should be regarded as belong- 

 ing to the branchial sac rather than to the mantle, since they are situated near the 

 endostyle inside the circular muscles of the mantle instead of outside, as are the longi- 

 tudinal mantle fibres proper. 



The dorsal or rapheal blood-vessel of the branchial sac is large (PL XIX, Fig. 15, 



