Chap. II. STRUCTURE OF THE ADULT. 53 



regularity in the ramificatioiis of these tubes, than of those which correspond to 

 the angles of the month, not only in their mode of ramification, but also in 

 their origin. Sometimes there are two independent ramifying branches on each 

 side of the middle tube, equally distant from it and from the simple lateral tubes, 

 whUe, at other times, there may be two independent branching tubes between 

 the middle tube and the lateral tube of one side, and only one on the other 

 side. At times, again, these branching tubes may be directly connected at their 

 base with the middle tube, either on both sides, or only on one side. But all 

 these irregularities are easily accounted for when it is recollected in what way 

 these tubes are formed, and their normal disposition may best be appreciated by 

 a comparison of younger specimens (as those of PI. XP. and XP.) with adults 

 (as those of PL VI. and VII.). In the young, in which the radiating tubes are 

 comparatively few, there are hardly any irregularities, and the radiating tubes cor- 

 resjDonding to the corners of the mouth form one bundle with a main stem and 

 more or less numerous branches from near the base, the main stem extending 

 straight to the peduncle of the eye, which is placed in the indentation of the mar- 

 gin, thus showing that the corresponding branching and anastomosing radiating tubes 

 of the adult arise from an increase of the branches and more frequent anasto- 

 moses among them, while the middle tube is enlarged without further branching. 

 A similar comparison of the tubes corresponding to the genital pouches shows 

 that at an early stage there arise three main branches from the genital pouches, 

 the lateral ones of which remain simple, while the middle one gives off bi'anches 

 from near its base, the middle stem, nevertheless, remaining simple while the 

 branches ramify again and again and form nvimerous anastomoses. As the gen- 

 ital pouch itself encroaches upon that main stem during its enlargement, the 

 result is that these branches appear in the end more or less independent from 

 the main axis. 



We have thus four simpler systems, with a single main central branch arising 

 from the corners of the mouth, and extending in the direction of the oral append- 

 ages to those four eyes in the marginal indentations, which are in the prolongations 

 of the same rays, and four more complicated systems arising from a triangular 

 sack, bordered on each side by a simple radiating tube, reaching the periphery 

 without further ramifications, and giving rise at their confluence with the marginal 

 tubes to but slight indentations, while the middle, simple branch terminates in the 

 peduncle of those eyes which occujjy the marginal indentations in the prolongations 

 of those rays in which lie the genital pouches. The obvious homology of these 

 parts, with those of Polyps and Echinoderms, enables us to introduce here a more 

 definite terminology to designate them ; for, as the radiating chambers are bound 

 by radiating partitions, on the margin of which hang the ovaries, thus alternating 



