PROCEEDINGS OF THE NATIONAL MUSEUM. 



693 



tbegillisfiiniisliedwithiiumeronsloiigitudmalmuscular fibers, at about 

 equal distauces apart, 

 wLich firmly connect tlie 

 upper edgesot" the lamell*. 

 The border of the inner 

 lamina where it lies 

 against the foot is defend- 

 ed, as above stated, by a 

 thin band of smootli tissue, 

 and where the plates join 

 this band their edges are 

 confluent. The same is 

 true of the edges of the 

 outer set wliere they im- 

 pinge upon the mantle. 

 The connection is very 

 brief and just at the ap- 

 pressed edge of the gill. 

 Each plate appears to 

 form a single blood sinus 

 or sac, as in NKcnla, with 

 numerous radiating mus- 

 <'ular fibers, as figured by 

 Mitsuliuri m Yohlia (Tab. 

 cit.,fig. 11). The main sur- 

 face is composed of con- 

 spicuously cellular epithe- 

 lium, as in JS^Kcula^ the edges are abundantly ciliated. The plates 



* Figure 1. Microtomic section of Euciroa pacifica, Dall, taken through the middle 

 of the foot, the lower ])art of the mantle lobes being omitted. From camera lucida 

 drawing from tlie origiual by J. C. McConnell, '{. 



I. Dorsal commissure of the mantle; II, II, blood sinuses connected with the 

 auricles of the heart; III, therectnm; IV, cavity of the nephi'idia; V, V, reticulated 

 connective tissue ; VI, VI, direct outer limbs of the ctenidia ; VII, VII, inner reflected 

 lamina-, and VIII, VIII, .inner direct lamina^ of the inner limb cut in a slightly oblique 

 section across the single plates; IX, IX, the palpi; X, the foot, more or less sur- 

 rounded by loose epithelial matter; XI, XI, lobes of the mantle; XII, XII, begin- 

 ning of the muscular region of the mantle lobes; incomplete below. 



Figure 1, A. Section of left cteuidinm of CaUorardia stearnsii, Dall, f. 



A. XI, CaUocardia, stem of the gill with bloodvessel; XII, inner direct and 

 reflected limb; XIII, outer direct and reflected limb in section. 



Figure 1, B. Side view of left ctenidium of CaUocardia stearnsii, showing outline 

 of the inner and outer limbs; the dotted line showing the limit to which the inner 

 reflected lamina of the inner limb rises on the side opposite that of the observer. 



B. XIV, point where the ctenidium is attached to the siphonal septum ; XV, ven- 

 tral extreme of the inner direct limb; XVI, outline of the outer direct and reflected 

 limb; the inner reflected laminip on the side next the body rises to the height indi- 

 cated by the dotred line. The single plates of which the gill-mass is composed are 

 not indicated. 



Fig. 1. 



MICROTOMIC SECTIONS OF EUCIROA PACIFICA AND 

 CALLOCARDIA STEARNSII * 



