284 CTENOPHORiE. Part II. 



wiile central cliymirerous cavity may be seen. On tlio actinal side, half the oral 

 tube is exposed, and on the margins its sections through the anterior and the 

 posterior interambulacra are visible as black specks. 



The course of the fluid in all these tubes is very easily traced. Starting from 

 the central chymiferous cavity, the main currents flow toward the actinal side of 

 the 1jody through the ambulacral tuljcs and empty into the circular oral tu1)e, but, 

 owing to the frequent contraction and great activity of the actinal region, the 

 progress of the fluid is constantly interrupted ; it then flows back and moves to 

 and fro in the ambulacral tul:)es, filling and distending to the utmost their rami- 

 fications in the spherosome, and thus distending it in the manner in which erectile 

 tissues are distended by capillary vessels. But when the obstacle arising from the 

 contraction of the actinal region is overcome, the fluid rushes into the circular 

 tube, from which arise also branches ramifying into the spherosome, and then runs 

 back through the coeliac tulaes into the central chymiferous cavity. Tlie total 

 absence of ramifications from the coeliac tubes into the spherosome or upon the 

 walls of the digestive cavity shows, that, in this type at least, the essential function 

 of the coeliac tul)es is not to provide the digestive apparatus with nutritive fluid.^ 

 In very young specimens these ramifications do not exist at all, and the chymiferous 

 tubes are as simple as in Pleuroliracliia ; but as they increase in size there arise 

 a few lateral Ijranclies, at first simple, then dividing (PI. I. F/'i/. G'' and G" magni- 

 fied), and then becoming more and more numerous and branching more extensively 

 so long as they continue to grow. 



The ovaries and spermaries stand in such close connection with the ambulacral 

 tubes and their ramifications, that they are best considered in this connection. In 

 very young specimens the ambulacral tubes are straight, simple canals ; but as they 

 advance in age, shallow pouches grow out of them upon the sides, increasing 

 gradually in size and expanding into irregular sacs, sometimes with a broad base 

 tapering gradually, at other times with a narrow base and expanding into irregular 

 vesicular sacs, usually, but not always, continued into slender ramifications pene- 

 trating into the spherosome. In these sacs the ovarian and spermatic cells are 

 developed; but, as already remarked, each ambulacral tube produces eggs in the 

 sacs of one of its sides and spermatic cells in the other : and while pigment cells 

 of a pale color line these sacs, superficial pigment cells of a deep pink color are 



the Clypeastruida', and forming a regular circular ■* Milne-Edwards represents the coeliac tubes as 



tube along the margin in the Scutellid*, are ho- ramitied in Beroe Forskali, Ann. Sc. Nat. 2d. ser. 



mologous to the lateral tubes of the Beroids brancli- vol. IG, PL VI. Fig. 1% b. It certainly gives off 



ing from the ambulacral tubes in the spherosome. no branches at all in our Idyia, nor have I seen 



In Echinoderms, however, the tubes send off suckers any such ramificatious of the ccnUac tubes in the 



similar to though smaller than the ambulacral tubes. other true Beroids which I have observed. 



