122 



The cavities in the head of the adult are very complicated; and, if I have understood 

 the original account correctly, they may be described as follows. — 



(I) a system of "Schnauzenhöhlen" in contact with the anterior end of the notochord. Of these, 

 which 1 may allude to as the praeoral cavities, there are four; two lying respectively along 

 the dorsal and ventral sides of the notochord, and the other two having a lateral position. 



(II) the "Stomocoel", coniposed of right and left halves, which are very unequally developed. 

 The ventral mesentery which separates the two cavities lies at the right side of the mouth, 

 which is thus, in the adult as in the larva, an organ of the left side. The right stomocoel 

 (loc. cit., fig. 23) has a dorso-ventral portion at the side of the true mouth, prolonged 

 forwards as a lonofitudinal tube which lies on the riijht side of the roof of the buccal 

 cavity. Ventrally the stomocoel passes into the right " epipterygial cavity", which lies on the 

 ventral side of the mouth and of the beginning of the pharynx, dorsally to the pterygial 

 or sub-atrial muscles. The left stomocoel is much more complicated. lts large epipterygial 

 portion is separated from the corresponding right cavity by a well developed ventral 

 mesentery, the asymmetrical position of which has already been noticed. The "outer lip- 

 cavity" is a direct anterior continuation of the left epipterygial cavity, and it occupies the 

 outer or ventral portion of the lip, which itself forms the floor of the (praeoral) buccal 

 cavity; sending a prolongation into each of the oral cirri. The "inner lip-cavity" occupies 

 the whole of the dorsal side of the lip, and is therefore close to the buccal cavity. It is 

 said to be in origin a part of the left stomocoel. The " velicavum", or coelom of the 

 velum, is continuous with the left .stomocoel in the larva, but in the adult it resembles the 

 inner lip-cavity in being completely closed. It is regarded as the left antimere corresponding 

 with the dorso-ventral portion of the right stomocoel, altered in position owing to the 

 assumption of a transverse position by the mouth, which in the larva opens on the left 

 side. Each epipterygial cavity is continuous with the "pterygocoel" or "Seitenflossenhöhle" 

 of its own side, a space which is better known under the name of the metapleural or 

 lymph-canal. 



(III) the "peribranchial cavity", consisting of the two dorsal spaces at the sides of the hyper- 

 pharyngeal groove, the unpaired ventral portion which underlies the endostyle, and the 

 coelomic spaces of the pharyngeal bars which connect the dorsal and ventral parts. The 

 coelom of the nrst pharyngeal bar differs from the similar spaces in the other bars by 

 running from the epipterygial cavities to the ventral or endostylar coelom of the pharynx. 



The comparison made by van Wijhe with the Enteropneusta gives the following results. 

 The dorsal praeoral cavity is not accounted for. The ventral cavity constitutes the whole of the 

 anterior body-cavity, since van Wijhe holds that the left head-cavity of other authors is not 

 part of the protocoel, but on the contrary is derived from ectoderm. The left "head-cavity" 

 gives rise, as is well known, to the "Raderorgan", or system of ciliated grooves of the buccal 

 cavity; and it is regarded as the representative of the stomodaeum of Craniata, and of the 

 dorsal tubercle of Ascidians. "Hatschek's groove", which is an anterior prolongation of the 

 Raderorgan, represents the sulnieural gland of Ascidians and the notochord of Hemichordata. 



