OLIGOTREMA PSAMMITES. 235 



foi'iu processes are moderately long on the outer (aboral) 

 surfaces of the arms, short over the remainder of the anterior 

 three-quarters of the body, long over the posterior quarter. 



Length of the body (antero-posterior), 17"5mni.; average 

 diameter of the body, 8*75 mm. ; measurement from tip to 

 tip of the expanded arms, 10"25 mm. 



Habitat — Lifii, New Britain, 50 fathoms. Only a single 

 specimen was collected by Dr. Willey. 



As I find it difficult, if not impossible, to give an intelli- 

 gible account of the anatomy of Oligotrema while retaining 

 the use of the term " mantle,'' and as the nomenclature of 

 the layers of which the Tunicate body is composed is in a 

 state of confusion in this and other respects, I must, before 

 giving a detailed account of the structure of the species, 

 enter into a short explanation of the morphological value of 

 the different layers, and propose some new terms which, I 

 hope, will tend to prevent obscurity. 



The different coats or layers of the Tunicate body are 

 generally described as follows, beginning from the outside : — 

 (1) The test (it has also been called the external tunic), which 

 is essentially a cuticular structure, much modified by the 

 presence of abundant intrusive mesoderm elements. Its 

 composition and relations are well understood, and require no 

 further mention in this place. (2) The mantle (also called 

 the inner tunic, body-wall, etc.). This sheet of tissue, 

 encircling the whole body, is covered on its outer surface by 

 a layer of epithelial cells, which is the external ectoderm. 

 Its internal surface is lined by an epithelial layer, also of 

 ectodermal origin, the atrial epithelium ; and between these 

 two there is a mass of tissue, consisting of a form of con- 

 nective tissue permeated by blood-spaces and sinuses, and 

 containing circular and longitudinal muscle-fibres, the 

 gonads, the renal organ, heart, stomach, intestine, etc., em- 

 bedded in it. The so-called mantle is separated by a con- 

 siderable space, the peribranchial or atrial chamber, from 

 (3) the branchial sac, the wall of which is made up as 

 follows : — Externally there is the atrial epithelium ; inside 



