NERVOUS SYSTEM OF THE POLYZOA. 303 



0"3mm. iu length ; aud it is so placed in the cell that the sid^' 

 on which the intestine is sitnated looks towards the distal 

 cnd_, and that on which the pharynx lies towards the origin 

 of the branch. When the polypide is strongly retracted, the 

 invaginated portion of the cell is directed obliqnely towards 

 the intestinal side, where it comes in contact Avith the middle 

 of the nninvaginated cell-wall. From this point the tentacnlar 

 sheath passes transversely towards the pharyngeal side, along 

 A\'hich it descends to the bottom of the cell. 



Attention to these positions, as well as to the direction iu 

 which the new cell-buds are formed, greatly facilitates tlu- 

 appreciation of the true position of parts in small fragments 

 as they lie in the field of vision of the microscope. The other 

 I'clations of the polypides do not concern the comprehension 

 of the colonial nervous system, to the description of which 

 I shall now turn. 



The nervous system of each branch consist of — l^v', a con- 

 siderable-shed ganylion situated at its origin ; 2dly, of a 

 nervous trunk running the entire length of the branch, at the 

 upper part of which it subdivides into brandies, going to the 

 ganglia of the internodes arising at this part, and 3dly, of a 

 rich nervous plexus resting on the trunk, and connecting the 

 ganglia just mentioned, as well as the basal ganglia of the 

 individual polypides. 



The basal ganglia of the branches (figs. 3 — 5 g) are placed 

 exactly at the line of separation between the primary and 

 secondary branches and the axis of the latter. They are 

 usually of a globular form, or slightly elongated and fusi- 

 form, and of a granular (minutely cellular ?) structure. Pale 

 and transparent in the youngest ramuscules, they soon 

 assume a faint yellowish colour, and lose their transparency. 

 In size they vary from 0*03 mm. in diameter (as measured iu 

 a very young twig, not more than 0"2 in length) to more 

 than 0"1 mm. 



From the basal ganglion a ne7've-trunk, of nearly uniform 

 thickness (from 001 to 0'05 mm., according to age), runs in 

 a straight line nearly to the end of the branch (figs. 3 — 5 s), 

 though not in its axis, but more or less near that side on 

 which the first row of cells is produced, and which may 

 briefly be described as the superior. The trunk is in most 

 cases single, but occasionally divided into two closely con- 

 tiguous or, in parts, slightly separated cords. More rarely 

 (in old branches) it is broken up, for a greater or less extent, 

 into a long-meshed plexus, composed of three or four prin- 

 cipal cords. The nerve is of a pale colour, and presents a 

 delicate, smooth contour. 



