34 



C. dodccalopims. 



The complicated arrangement of the arms in this species can be explained by comparison 

 with C. gracilis. I defer considering the account which Masterman (98, 2, p. 521) has given 

 of this subject until I have described the arms of C. levinseni. 



The fundamental arrangement of the arms of Cephalodiscus is most easily seen in the 

 buds, and is shewn in Masterman's figs. 25, 24 and 27 (PI. II) of the paper just referred to, 

 or in my own figures (PI. III, figs. 32, 30 ; PL I, fig. 4) of the buds of C. gracilis. The arms 

 are developed as a simple linear series of outgrowths, at the sides of the central nervous system, 

 from the dorsai extremity of the collar. The most anterior arm is developed first, and the others 

 in resfular order from before backwards. This arrangement is but little disturbed in the adult 

 C. gracilis (figs. 52, 53). 



Figs. 151 — 141 (PI. XII) represent every second section of a series of C. dodecalophus, 

 and readily admit of comparison with figs. 43 — 53 of C. gracilis. The plane of the sections 

 may be described as obliquely sagittal, the first section (fig. 151, which is more highly magnified 

 than the others) pa.ssing through practically the middle line of the proboscis, at the front end 

 of the central nervous system, and through the extreme outer edge of the collar-canal on the 

 ventral side. As both sides of the metasome are, however, cut there was probably some rotation 

 of the anterior part of the body in the neck-region. 



Fig. 151 shews the external opening of the left proboscis-pore (/././.), the anterior 

 dorsai horn of the left collar-cavit\- (ó.c.-a.), the tip of the notochord {tic/i.), and the dorsai 

 collar-mesentery, connecting the notochord with the central nervous system. The collar-cavity 

 {d. c.- r.) is cut along nearly its entire length, the section passing to the right of the mouth. 

 At the ventral end of the cavity is seen the outer edge of the collar-canal (c. c. e.), which lies 

 in a recess of this region of the collar-cavity. Immediately behind the collar-pore is the strong 

 nerve-tract or lateral nerve (/. u.) passing from the central nervous system {c. n. s.) to the stalk. 



The succeeding figures represent sections lying further to the right than the one which 

 has just been described. Fig. 150 has nearly reached the superficial groove which separates 

 the collar from the metasome. The lateral nerve is seen passing from the central nervous 

 system, behind the collar-cavity, to reach the ventral surface. The extreme tip of the notochord 

 (nch^ is cut by this section; immediately in front of it being the pericardium {per.), the internal 

 opening of the left proboscis-pore (/./. /.) and the anterior end of the left collar-cavity [b.ci-a.), 



Fig. 149 shews the last of the connexion between the collar and the metasome. The 

 position of the collar-pore in the preceding sections is now overhung by a lateral lobe of the 

 operculum [op. r.), developed as in C. gracilis, its morphologically external wall being cut 

 tangentially in this section. 



In fig. 148, the lateral lobe of the operculum is cut so as to exposé most of its cavity, 

 which is separated by a ridge of the basement-membrane (cf. fig. 153, b. m.) ircm the rest of 

 the collar-cavity. The inner edge of the right proboscis-pore [p.p.r.) is cut tangentially. 



In fig. 147, the lateral lobe of the operculum is separated from the arm-base (a./;.), which 

 is the more dorsai part of the collar-edge. The external opening of the right proboscis-pore is 



