692 DEEP WATER MOLLVSES AND BRACEIOPODS—DALL. vol. xvii. 



dorsal pal]>us, which is more or less attached to this blister-like body. 

 On cutting the tissues so as to expose the parts it is seen that the 

 sacs form part of the dorsal palpi which are largely adnate upon the 

 posterior faces of the sacs, with the free extremity recurved and coiled 

 as in the figure* The sacs when opened appear empty and thin 

 walled, resembling blood sinuses. The palpi are not distinctly cross 

 striated, but are more or less folded, like a book, upon themselves. The 

 ventral palpi are long, slender, and nearly smooth. Between the bases 

 of these projects a sort of lappet of cuticular tissue, broad, tiattish, 

 bifurcate behind and lying against, but not attached to, the anterior 

 edge of the upper part of the foot. Above it the mouth is visible as a 

 narrow slightly arcuate slit. I have not observed before anything 

 exactly corresponding to this lappet in any i)elecypod I have examined 

 or found mentioned in the literature. What the office of the sacs in 

 connection Avith the palpi may be I can not imagine, unle> s, when filled 

 with tiuid, their contraction may erect the tissue of the palps. 



The most interesting part of this investigation relates to thectenidia. 

 These resemble in construction the archaic gills of YohJia, SoJemya, 

 etc., with interesting differences. Behind they are firmly attached to 

 and continuous with the broad siphonal septum; on each side and in 

 front their outer edges are firmly soldered to the mantle. The inner 

 edges on each side of the foot are confluent near the base of insertion 

 and bordered by a smooth band of connective tissue which is closely 

 appressed to, but not organically connected with the foot, which passes 

 between them. These edges behind the foot, however, are united to 

 each other by delicate yet firm tissue not easily ruptured. Looking 

 down upon this surface, beside the median line of junction it is seen to 

 be marked by two imj)resned grooves on each side between which, 

 obliquely waved, extend the edges of closf^ly appressed plate-like 

 lamelhie. On cutting the gills transversely it is found that these plates 

 present much the appearance of the same organs in Yoldia limatuJa as 

 figured by Mitsukuri,t but with important differences. The fibrous 

 suspensory tissue, by which thectenidia are connected with the mantle, 

 forms a narrow band extending obliquely at an angle of 35° to 45° from 

 the vertical plane of the body, when it is perforated by a large vessel 

 running longitudinally. Morphologically below this, but actually 

 obliquely outward, is a band of smooth tissue separating two sets of 

 lamellne. These lamellaj are not equal and symmetrical as in Nueula, 

 nor are they set at riglit angles to the stem of the gill, but trend 

 obliquely backward on each side like the vanes of a feather. The 

 outer set of lamell;^ are wider from side to side and shorter verti- 

 cally than the inner set. The latter are separated by a narrow mem- 

 branous band from a third set, forming an ascending or reflected 

 lamina, for which I was unable to detect any main blood vessel com- 

 parable to that of the main stem of the gill. The upper surface of 



* Compcare pi. xxiv, fig. 5, p. 



t Studies from Biol. Lab., Johns Hopkins Univ., ii, pi. xix, fig. 11, 1882. 



