42 Prof. Mcintosh's Notes from the 



being joined to the body-wall by a firm isthmus in the 

 middle line, its two surfaces beyond being structurally 

 differentiated, the inner (that is, next the body-wall) being 

 coated by a thick layer of the long hypodermic cells with 

 the nuclei near the surface, whilst the outer has much 

 shorter cells, the inner ends of which seem to run into the 

 reticulated connective-tissue of the central region. Masses 

 of gland-cells, moreover, occur along the convex margin of 

 the jacket. In the area of the cephalic ganglia the modified 

 hypoderm is thickened in the mid-dorsal line and also 

 laterally so as to form a protection to the organs. Then 

 on the side (generally the left) from which the opercular 

 stalk springs this modified hypoderm bulges out and envelops 

 it (PI. IV. fig. 20). Further, the glandular nature of the 

 ventral wall diminishes, and a split separating the jacket or 

 collar appears and joins the folded lateral and dorsal flaps, 

 both the inner surface of the collar and the outer of the 

 body-wall being invested by layers of the hypoderm. As soon 

 as the collar becomes free (in section) the entire body-wall, 

 with the exception of a narrow lateral belt on each side, is 

 invested by this modified hypoderm, the thickest parts being 

 the dorso-lateral and mid-dorsal regions ; and the origin of 

 the opercular stalk has the same investment, special support 

 being afforded by the adjoining mid-dorsal and lateral en- 

 largements of this modified hypoderm. Proceeding forward 

 the ventro-lateral regions of this coat are considerably 

 thickened, and a deep furrow now cuts off the opercular 

 stalk (PI, IV. fig. 22). The diminished area of the anterior 

 region is specially stiffened, for in section the greater part 

 of its surface is composed of this modified hypoderm, the 

 only gaps being those of the mouth, the branchial trunks, 

 and a coelomic space. The shape in section is that of a 

 curved dumb-bell (PI. IV. fig. 23), the narrow median region 

 with the oval slit corresponding to the handle and the 

 enlarged lateral regions to the bells. Instead of the dorsal 

 region having the thick layer of modified hypoderm, it is 

 now the ventral surface, and the band is dilated at each side, 

 after which is a connective-tissue belt, then a band of the 

 modified hypoderm round the bulbous ends, in which 

 by-aud-by appear the slits indicating the separation of the 

 branchial filaments. These slits have a regularly arranged 

 cellular investment with distinct nuclei, and they increase 

 in size and number from behind forward. The intermediate 

 region, between the dilated ends of the dumb-bell, has only a 

 thin coating of ordinary hypoderm, and is thus in contrast 

 with the lateral regions. Advancing forward a slit appears 



