NEW POINTS IN THE STRUCTURE OF AMPHIOXUs. 267 
Moreover, in the young Elasmobranch, as in the adult 
Amphioxus, the excretory canals placing the coelom (pharyn- 
godorsal celom of Amphioxus, pleuro-peritoneal cavity of 
shark) in communication with the exterior, are open involu- 
tions of the wall of the atrial chamber or epiceel, and there- 
fore aboriginally of the ectoderm. 
Before discussing further the homologies which are thus 
suggested (L say advisedly ‘ suggested’ as appropriate to such 
speculation), it seems most desirable that possible sources 
of direct confirmation or destruction should be thoroughly 
interrogated ; above all, the ontogenetic forms of the Lam- 
prey. 
I subjoin a list of the new (and of some of the old) terms 
which have been used in this paper. 
1. Atrium: the space enclosed by the downward growth of two folds, one 
running along the greater part of each side of the embryo. The 
preoral portions of the two folds do not meet below; they partially 
enclose an open space, the buccal cavity or preeoral atrium. The 
post-oral portions of the folds, which are called ‘ epipleura,’ coalesce 
in the middle line, and form the post-oral atrium or epica/—the 
chamber into which the pharyngeal slits open. 
2. Atriopore : the posteriorly placed ventral aperture, by means of which 
the post-oral atrium communicates with the exterior. 
3. Atrial tunic: a brown pigmented membrane, which lines the sides of the 
atrium and of the organs suspended in it. 
4. Colom: the primitive body-cavity found in the Worms. 
5. Entero-cel: a body cavity, formed by a pinching off of the primitive 
enteron, or alimentary canal. 
6. Epical : a body-cavity, formed by the outgrowth and coalescence of folds 
of the primitive body-wall. 
. EZpipleura: the lateral down-growing folds which, by their coalescence in 
the middle line below, form the atrium or epiceel. 
8. Metapleura : the upstanding hollow ridges or latero-ventral folds which, 
in Amphioxus, form the lateral margins of the ventral surface. 
9. Pharyngo-dorsal cvlom: that portion of the ccelom which exists on the 
dorsal surface of the pharynx on either side of its attachment to the 
notochordal sheath. 
10. Pharyngo-pleural septa: the double folds by means of which the upper 
part of alternate pharyngeal bars are attached to the epipleura. 
11. Pharyngo-pleural interspaces: tle open spaces between the septa, com- 
municating with the pharyngeal cavity and with the atrial chamber. 
12. Postatrioporal: that which is posterior to the atrial pore. 
13. Schizocel: a body-cavity formed by the splitting or separation of the 
primitive muscular body-wall (somatopleur), and primitive muscular 
gut-wall (splanchnopleur). 
