Primitive Cell-layers of the Emhryo. 333 



worms possess in their vascular system the commencement of 

 a body-cavity. The most conclusive evidence which can be 

 adduced on the matter is the analogy of such a mollusk as Phyl- 

 lirhoe^ where, as in other Mollusca, the perivisceral cavity is de- 

 veloped only as a series of sinuses, of which the pericardium is 

 one. or where, as we may say, the perivisceral space is reduced 

 to ih.e pericardium. This pericardium is produced at one end 

 into a tube or canal ciliated at one part, which opens to the 

 exterior. The ciliated tube represents a segment-organ, as 

 must be admitted for the renal organ of Mollusca generally, 

 and especially for the so-called "hearts" or "oviducts" of 

 Brachiopoda. In Pliyllirlioe we have, it seems to me, as in 

 the flat-worms, the imperfect channellings and spaces of a 

 "parenchymatous" body placed in relation with the exterior 

 by the segment-organ, the wall of which is not discontinuous 

 with that of the channels. It is when the perivisceral space 

 becomes large and expanded that the segment-organ floats in 

 it with a trumpet-like inner orifice ; on the other hand, when 

 the blood-lymph-space is canal-like^ then the segment-organ is 

 merely its continuation to the exterior. 



Ciliation and contractility, both exhibited by the "water- 

 vascular system" in Trematodes, are both familiar characters 

 of the perivisceral space when developed on a more capacious 

 scale. Contractility is of course in the nature of the case, the 

 walls of the perivisceral space being muscular. Cilia occur in 

 the perivisceral cavity of some Chojtopoda and in that of 

 Gephyrea, in the primitive mesoblastic cavity of the developing 

 Lamellibranch Pisidium and of Aplysia^ also in the peritoneal 

 (perivisceral) space of the frog. 



The condition of the vascular system in diiferent genera of 

 leeches is instructive, tending, as it seems, to bridge over the 

 gulf between a simple perivisceral primitive blood-lymph-space 

 and the more complicated differentiations of lymphatic systems, 

 pleuro-peritoneal cavity, and blood-vascular system to whicli 

 it simultaneously gives rise in higher organisms. The blood- 

 lymph-space exists in the common leech as four chief longitu- 

 dinal canals, in one of which the nerve-cord lies. The apertures 

 of the segment-organs lead into closed pouches, whose cavity is 

 also to be reckoned to the blood-lymph-space, though not in 

 continuity with its longitudinal portions. In other leeches 

 (e. g. Branckiobdella), whilst two of the longitudinal canals are 

 retained, excavation is carried on in the mesoblastic parenchyma 

 in such a way as to leave the segment-organs floating trumpet- 

 like in a great perivisceral sinus, in which also the nerve-cord 

 lies. The longitudinal canals may, as in Ilirudo, contain a 

 liquid impregnated with luemoglobin, and remain closed from 



