Gatty Marine Lahoralorrj , St. Andrtws. 241 



at this level all round, thoufih it is thickest ventro-Iaterally, 

 and it was this layer whieli was first encountered in front. 

 The structni'e of this lateral band of so-called basement- 

 tissue closely resembles that of the central system, and it is 

 possible that it may ])erform functions of a sensory kind in 

 connection with the lateral flaps of the vestibule. The flaps 

 gradually unite to form the lower half of the A'estibnle, the 

 thicker band of j)ale tissue still being retained ventrally 

 with a thin connecting-band dorsally. Proceeding a little 

 backward the main nervous band disappears from the dorsum 

 and is confined to the lateral regions of the body-wall, from 

 which it gradually thins off dorsally. Finally, Avhen the 

 lip-organ appears in the section over the oral gap, a large 

 nerve-cord alone is left at the lower limit of the former nerve- 

 band (PI. VII. fig. 3, nc), all the hypodermic layer dorsad of 

 it having assumed the usual condition. In sections of Sacco- 

 c/rrw5 near the mouth Dr. Goodrich * found the lateral cords 

 (his " oesophageal commissures ") in a similar position. This 

 limited nerve-area presents in section ])ale, finely granular, 

 transverse striation, tlii'ough which delicate fibrils from the 

 hypoderm external to it pass to the basement-tissue. When 

 the circle of the body-wall is complete — that is, immediately 

 behind the oral gap (PI. VII. fig. 3), — the large nerve-cords 

 are situated a little below the middle line of the body-wall, 

 and have a blood-vessel in the muscle to their inner side. 

 The central region is still lined by hypoderm, and the thick 

 pale baud of the inner layer is iufero-laterally conspicuous. 

 Then the hypodermic layer of the vestibule passes into ttie 

 gullet, and sections of the lip-organ {Ip.) appear, whilst the- 

 hypodermic inner lining of the dorsal region is shut off by a 

 deep fold with a narrow layer of hypoderm from the vesti- 

 bule, the rest of the large arch above having a thick coat of 

 the same tissue. Externally, again, a change has occurred 

 in the mid-ventral line, for the thick lateral coat of hyi)o- 

 derm in which the nerve-cords lie has thinned off ventrally, 

 leaving a considerable area with just a trace of it ; but thi» 

 appears to occur only for a short distance. With the 

 termination of the vestibule lined by hypoderm, and the 

 increase of the lip-organ in section, the ventral hypoderm of 

 the body-wall again gradually thickens from the middle line 

 outward. Moreover, the narrow pocket formed by the 

 first septum lies on each side of the lip-organ (PI. VII. 

 fig. 4), and then is quite shut off from the upper cavity 

 (vestibule) lined by hypoderm, and which represents the 



* Quart, Journ. Micr. Sci. n. s., vol, xliv. 



