50 Prof. M'Intosh's Notes from the 



the larger masses beneath them, and they soon fuse with 

 them, meanwhile this wide space dorsally intervening. The 

 diminution of the cavity of the thoracic gland on each side 

 permits the muscles to assume a more oblique position, so 

 that their axis in section is directed downward and outward. 

 On the disappearance of the thoracic glands (in the progress 

 backward) the muscles more closely approach each other in 

 the mid-dorsal line, the upper as well as the lower ends 

 being pointed in section. Then a tendency for the lower 

 ends to bend inward is noticeable, the investing mesentery 

 being still visible externally, whilst the muscles have like- 

 wise considerably increased in bulk. This divided condition 

 of the dorsal longitudinal muscles characterizes the anterior 

 region of the body, for toward the middle there is complete 

 union of the halves (PI. V. fig. 27), and the entire muscle 

 has greatly increased in size, forming a broad crescent which 

 reaches by its expanded inferior edges almost to the ventral 

 surface. No distinct trace of a mid- dorsal fissure is seen, 

 the median mesentery being attached to a slight muscular 

 ridge at its inner surface. 



Alimentary Canal. — The various ciliated grooves from the 

 branchial apparatus to the mouth converge to the double 

 isthmus connecting the two fans, and which in the sections 

 is usually V-shaped, the apex being directed ventrally 

 (PI. IV. fig. 23), the upper layer being pierced by a 

 blood-vessel at each end. Then, proceeding backward, the 

 V expands into a curve, the ventral isthmus receives a 

 coating of hypoderm, both isthmuses becoming shorter and 

 thicker, with a slit at either end opening by-aud-by 

 to the dorsal surface. Further, the cellular walls of the 

 central chamber of the isthmus (the vestibule) have a more 

 finely granular structure than the hypoderm" covering the 

 ventral surface, and the dorsal border is soon modified, by 

 a median furrow, into two thick ridges — about the level of 

 the origin of the stalk of the operculum. The dorsal wall 

 of the vestibule or mouth increases in thickness, and the 

 opercular stalk sends out a process which fuses with the 

 opposite side, so that two apertures now exist, viz., the mouth 

 and that dorsad of the groove and ridges and formed by the 

 external pit. Processes fuse with the point of junction, and 

 others from the dorsal region of the now irregularly quad- 

 rangular part soon fill up the extended area (PI. IV. fig. 21), 

 leaving a small space dorsad of the mouth with its ventral 

 edge marked by the groove before-mentioned, and showing a 

 slight differentiation of its hypodermic wall. The vestibule, 

 on the other hand, has glandular walls which stain deeply 



