244 GILBERT C. P.OURNE. 



to the " mantle " of previous authors, and an inner layer, the 

 endoplerome, surrounding- the gut. The inner surface of the 

 ectoplerome and the outer surface of the endoplerome are, of 

 course, covered by atrial epithelium. As the atrial cavity 

 does not extend to the extremities of the body, there is pos- 

 teriorly a solid mass of undivided plerome in which part of 

 the stomach and testis are embedded, and anteriorly the 

 plerome is undivided and is produced into six nniscular 

 processes which are the six circiimoral arms. 



The bulk of both the ecto- and endoplerome, as well 

 as the anterior and posterior undivided portions of the 

 plerome, is made up of a form of connective tissue consisting 

 of a structureless or sometimes slightly fibrillated matrix in 

 which a few corpuscles are embedded. This matrix stains, 

 like all intercellular substance, with picro-nigrosin and picro- 

 indigo-carmine, and its characters are fairly well shown in 

 figs. 22, 26 and 30. The connective-tissue matrix is honey- 

 combed with irregular cavities, and the muscles and all the 

 principal organs of the body are, as it were, embedded in it. 

 The blood-spaces have no regular arrangement, and I could 

 not find any trace of definite blood-vessels such as occur in 

 most simple Ascidians, unless a delicate pavement epithelium 

 lining some of the larger cavities near the heart might be 

 taken as an indication of regular blood-channels. 



Musculature. 



All the muscles of the animal are contained in the ecto- 

 plerome or in the undivided plerome of the anterior end. 

 There is no trace of a layer of muscle-fibres surrounding the 

 gut or the branchial sac. The general arrangement of the 

 muscles is shown in figs. 3 and 4. The musculature is 

 weak, except in the regions of the atrial and branchial 

 siphons and the arms. Over the greater part of the body it 

 consists of a series of thin longitudinal bands, placed rather 

 far apart, and these are crossed at right angles by a series 

 of circular bands, the two series forming a rectangular 



