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



apophyses are developed in pairs, growing symmetrically on both sides of the ring. 

 The most important of these apophyses are : — (l) basal apophyses, arising from the 

 basal pole of the ring ; (2) mitral apophj^ses, arising from the apical pole ; (3) dorsal 

 apophyses, arising from the middle of the dorsal rod ; and (4) ventral apophyses, 

 arising from the middle of the ventral rod. The two former arise in the principal axis, 

 the two latter in the sagittal axis of the body. Very frequently the latter pair is 

 replaced by two paii's of transverse branches, one inferior (mandibular) and one superior 

 (orbital). The apophyses of the ring are either simple or branched, often very large, 

 richly ramified, and give origin to a number of further products. 



Whilst in the Stephanida the primary sagittal ring alone represents the whole 

 skeleton, it produces in all other Stephoidea one or more secondary rings. The 

 most important of these is the horizontal basal ring, appearing first in the Semantida 

 (PL 92). From the base of the sagittal ring there arise in the horizontal basal plane 

 two pairs of lateral branches or " basal apophyses." The curved opposite branches of 

 the corresponding pairs become united on each side of the primary ring (right and 

 left), and so produce a second, horizontal ring, perpendicular to the former. This basal 

 ring encloses two paired basal gates, which are enclosed on the medial side by the basal 

 rod of the sagittal ring, and on the lateral side by two united apophyses (Semantis, 

 PI. 92, figs. 1, 2). These two primary basal gates are of the greatest morphological 

 importance; we call them the "jugular gates or jugular pores " (in the descrij^tion of 

 Biitschli, the pores I, loc. cit., p. 498). Tlie dorsal jaair of basal apophyses (on their 

 posterior edge) are the coracal rods, e (rods e of Biitschli) ; the opposite ventral pair 

 (on their anterior edge) are the clavicular or furcular rods, y (rods el of Biitschli); compare 

 Pis. 92-95, and their explanation. 



The skeleton of Semantis, the prototype of the Semantida, thus assumes the 

 characteristic form of a signet-ring. The basal ring enclosing the two jugular pores 

 corresponds to the seal-plate. It is commonly more or less horizontal ; but often the 

 apophyses descend obliquely (PL 92, figs. 1, 2, 13, &c.), more rarely laterally 

 (PL 29, fig. 11). The further development of this typical form is essentially efi"ected 

 by the production of new basal pores in the horizontal seal-plate. In Seinantrum 

 (PL 92, figs. 3, 4, 5) we find already four basal gates. Behind the jugular gates is 

 formed a second pair, the " cardinal gates " (pores II of Biitschli). These are enclosed 

 on the anterior margin by the coracal rods (e), on the posterior margin by the scapular 

 rods (d), a third pair of basal apophyses, arising behind the former from the sagittal 

 ring and uniting with them (the rods e2 of Biitschli). Commonly the two posterior, or 

 cardinal gates are much larger than the two anterior, jugular gates. This characteristic 

 basal plate of Semantrum, with two pairs of basal pores, is of the greatest morphological 

 importance, as it is inherited in by far the greater number of the Nassellaeia, though not 

 so generally as Biitschli supposes. The basal ring of Semantrum is either more circular 



