OLIGOTKEMA PSAMMI'IES. 237 



wall^ and contains most of the important viscera^, as well as 

 the museuhiture and the nerve-ganglion. It may most cor- 

 rectly be described as the external part of the body-wall, 

 which has been, as it were, accidentally cut off from the 

 internal part surrounding the pharynx (and in Oligoti'ema the 

 oesophagus) by the ingrowth of the atrial and peribranchial 

 cavities. 



In the light of our existing knowledge of development and 

 comparative anatomy, a Tunicate must be regarded as an 

 animal in which the primitive coelom has been obliterated 

 and its place taken by a mass of coniiective tissue permeated 

 by an irregular system of blood-spaces and sinuses (liEenio- 

 cnele). In the adult the cavities of the gonads, and possibly 

 the pericardial cavity ^ and the lumen of the renal organ, are 

 the only representatives of the original coelom. Considered 

 from this point of view the Tunicate is a solid animal, — that is 

 to say, the space between the gut and the external epithelium 

 is theoretically filled up with mesodermic tissue, except for 

 the relatively small cavities of the gonads, the pericai'dium, 

 and renal organ. To this mesodermic tissue I propose to give 

 the name plerome, from TrXripMfia, a thing filled up." 



If we imagine the formation of the atrial cavity to be 

 delayed to a much later period of development than is 

 actually the case, a transverse section taken midway through 

 the body of an Ascidian, before the establishment of the atrial 

 cavity, would have the characters shown in the annexed 

 diagram. Fig. 1. The area shaded with dots represents the 

 plerome. The atrial cavity would be formed from two dorso- 

 lateral invaginations of the ectoderm, which would become 

 confluent in the dorsal region and give rise to a flattened 

 sac, overlying the dorsal aspect of the pharynx and opening 



' The pericardial cavity of Ascidians, being formed I'lom the ventral wall of 

 the pharynx, may possibly be regarded as an independently develojied portion 

 of the enteroccele. 



It may be objected that this term is already used by botanists. But, its 

 botanical connotation is so different, from that, now proposed, that it is unlikely 

 that any inconvenience can arise. 



