RETORT ON THE TUNICATA. 229 



it two pairs of oblong plates (PI. XXXVII. fig. 2, t.t.) form a stiff shield for the principal 

 viscera, which lie upon them. Two series of oval plates range on either side of the larger 

 median ones, and extend up as far as the exhalent aperture. The test tunic is continual 

 downwards from the upper region of the body to form the outer wall of the cylindrical 

 stem, thus forming a tube. The lower end of this tube is widened out into a funnel- 

 shaped mouth, and in the specimen had apparently been torn away from some object of 

 attachment. 



"The substance of the test is composed of transparent hyaline tissue, in which are 

 embedded small bodies, the larger of which have a length of from *007 to '014 mm. 

 They are irregular in form, sometimes crystalline, or with apparently crystalHne contents. 

 They are not sensibly altered in appearance by the action of acetic acid, and no efferves- 

 cence is produced in the test tissue by that re-agent. The bodies are present in the 

 greatest abundance in the test-tissue at the base of the stem. In the plates of denser 

 tissue they are rather less abundant than elsewhere. The test forms a simple sac, 

 continuous with the tubular cavity of the stem. 



" The exhalent orifice is an aperture in the test, situate at the end of a short tube 

 projecting externally on the dorsal aspect just below the nerve ganglion. Into it the 

 ducts of the generative glands and the rectum open. The inhalent aperture was entirely 

 obliterated in the only specimen obtained ; it must have lain on the ventral aspect of the 

 body, since the dorsal wall was intact. The arrangement of the muscular fibres and 

 remnants of attachment of the gill sac seemed to indicate the position for it given in the 

 figure, where it is introduced conjecturally (PI. XXXVII. fig. 1, br.). 



" Closely attached to the inner surface of the test-wall is a delicate tunic (the mantle) 

 containing muscles. The muscles occur in the form of very fine bands, which have a 

 nearly parallel course. The series of bands springing from near the region occupied by 

 the heart, follow the curved inner surface of the test-cavity towards its superior margins. 

 The muscles are disposed most thickly in the lateral regions. The mesial region of the 

 dorsal surface is entirely devoid of them, but they extend over the whole ventral wall. 

 Only the lateral fibres are indicated in the figure ; they are prolonged superiorly in a 

 horizontal direction along the upper margin of the dorsal wall of the test-cavity. The 

 tubular cavity of the stem is filled by a core composed of muscular fibres embedded in 

 gelatinous tissue, a prolongation of the mantle. 



" Of the gill sac only a small portion remained intact in situ, but fragments here and 

 there attached indicated an arrangement as shown in the figure. The small portion 

 in situ lay over the nerve ganglion. The fenestrations in the membrane are small, simple, 

 and irregular (PI. XXXVII. fig. 4). 



"The mouth is situate nearly in the middle line (fig 1, ce.a.). It leads by a short 

 transparent oesophagus to a stomach (st.), which has opaque walls corrugated externally ; 

 and this viscus leads into a rectum which curves up to end at the exhalent aperture. 



