28 THE VOYAGE OF H.M.S. CHALLENGER. 



presenting from one to three or more buds at various stages, and in all cases these, 

 proceed from the terminal region of the pedicle. 



The early buds consist of minute and, in the preparations, somewhat pale clavate or 

 pyriform bodies attached by the narrow end to the pedicle. They are situated round oi' 

 near the tip (which has the thickened investment), and are observed as little processes 

 projecting from the hypoderm. They appear to rise close above the terminal disk, and, 

 in their earlier stages at least, seem to have a channel of communication with the pedicle 

 of the adult (PI. VI. fig. 1, ch). This channel runs upwards along the stalk of the bud 

 as far as the downward bend of the hypoderm (shield) where it is lost. In the earlier 

 buds longitudinal sections, for an examination of the best of which I am indebted to Mr. 

 Harmer, show, very clearly the three regions of the body, viz., shield, nuchal region and 

 body-proper, and each has its special spaces. Thus the shield presents its single cavity, 

 the nuchal or collar-region its paired spaces, and the third region contains the body- 

 cavity divided by the mesenteries (dorsal and ventral). 



Very soon the pedicle is differentiated from the disk ; the young animal consisting of 

 a large, flat, and somewhat thick disk, and a short, broad and somewhat crenate pedicle 

 attached to the parent. Two ovoid opaque thickenings are observed about this stage 

 dorsally (PI. V. fig. 3, Ip), and these are the earliest indications of the lophophoral 

 plumes. In cross section from above downwards the shield consists of a thick layer of 

 hypoderm which attains its greatest development ventrally, for this coat is in the form 

 of a flattened ring round the central chamber. The latter is bounded by a firm investment 

 of basement- tissue having fibres internally. It is this chamber in the younger forms that 

 communicates with the pedicle of the adult. The two dorsal bosses are simply hypodermic 

 growths of the upper layer of the shield, and present an elevation between them. 



At a somewhat lower level the elongated cavity in the centre of the shield gives place 

 to a rounded median chamber (PL V. fig. 3, vc), with the dorsal hypodermic mass above 

 it ; while the great bosses of the plumes lie on each side. The latter still present the 

 same structure as the hypoderm of the broad disk, though there is a faint indication 

 of a linea-r streak in the centre. In the next slice the two nuchal or collar-spaces appear, 

 with the alimentary canal in the centre. In the septum, between the collar-spaces, the 

 tip of the notochord is observed, and it appears to be larger proportionally in the young 

 than in the adult. The basement-tissue bounding the nervous centre is now defined, 

 and the shield and bosses are diminishing. Behind this the folds of the alimentary 

 canal fill the central region of the body, the intestine being especially distinct as a 

 median canal, the longest axis of which is directed ventrally instead of being transverse 

 as in the adult. Posteriorly the body presents, firstly, two lateral chambers with fibres 

 internal to the basement-layer, and secondly, a median (dorsal), with a process of 

 basement-tissue running to the ventral line, and forming the septum between the two, 

 former. The pedicle seems to be filled with muscle-cells. 



