ON THE DIPLOCHOKDA. 717 



These fibres make up the main nerve-mass lying over the sub- 

 neural gland ; they are seen in transverse section in figs. G — 

 8, and in longitudinal section in figs. 1 — 5. Forwards they 

 run along the dorsal surface of the buccal shield^ and back- 

 wards they branch outwards to form the two lateral cords; 

 hence, in longitudinal sections such as figs. 1 — 5, the nerve- 

 fibres appear to terminate abruptly backwards against the 

 wall of the pharynx, which here is in contact with the dorsal 

 ectoderm. The same remark applies to the dorsal blood- 

 sinus. 



Immediately in front of the central nerve-mass is seen a 

 pit or depression, the ectodermal pit (fig. 1). This pit lies 

 exactly over the apex of the subneural gland, and extends 

 transversely in a slightly crescentic form. At the outer ends 

 of the crescent open the pre-oral pores, which are situated 

 at the posterior termination of the pre-oral cavity, at the 

 level of the central complex. The ectodermal pit, therefore, 

 represents the line of division dorsally between the head or 

 buccal shield and the collar, as do the " epidermistache" 

 of Balanoglossus and the epiblastic pit of Actinotrocha 

 (formerly termed the neuropore). 



It is well to notice in dealing with this part that the 

 nerve-mass is co-extensive with the collar, as this part is 

 considerably narrower in the dorsal region ; and further, that 

 the subneural gland lies entirely in the collar area. At first 

 sight one is inclined to suppose, reasoning from prior know- 

 ledge of Balanoglossus, that the two collar cavities are 

 produced forwards into the pre-oral cavity, but such is not 

 the case. The buccal shield is produced backwards ventrally, 

 but the subneural gland lies in its primitive position in the 

 collar, and is in no way produced into the pre-oral cavity. 

 It is a backward ventral extension of the buccal shield 

 which makes the subneural gland lie in front of the mouth, — 

 not, as in Balanoglossus, a forward median extension of 

 the subneural gland into the pre-oral cavit3^ 



2. The Subneural Gland and Pharynx. — The sub- 

 neural gland is an elongated cfecal tube or prolongation of 



VOL. 4G, PART 4. NEW SERIES. AAA 



