82 THE VOYAGE OF H.M.S. CHALLENGER. 



Tlic Test is clear and transparent, and is thin all over the body. 



Tlie Mantle has the musculature very distinct. The first and second bands join on 

 the sides of the body, but are incomplete dorsally. The third, fourth, fifth, sixth, and 

 seventh bands are distinct from one another, and are only interrupted in the median 

 ventral line. The third band approaches the fourth on the mid-dorsal line, but does 

 not fuse with it. There are short curved bands at the ends of the transversely 

 elongated branchial and atrial apertures, and there are also a few very slight short 

 muscle bands in the median dorsal line immediately in front of the atrial aperture. 



TJie EndostyU is conspicuous and straight. It extends forwards in front of the 

 branchial aperture. 



The Dorsal Lamina is rather narrow. It is attached as far. back as the fifth 

 muscle band. 



The Dorsal Tubercle is very simple, and of elliptical outline. The nerve ganglion 

 is placed immediately behind it. 



Tlie Visceral Mass is unusually small. 



Locality. — March 16, 1875; Station 222, north of the Admiralty Islands, Pacific; 

 lat. 2° 15' 0" N., long. 146° 16' 0" E. ; surf. temp. 82°-8 ; one specimen. 



This little specimen from the surface of the tropical Pacific is in external 

 appearance somewhat like the aggregated form of Salpa democratica-mucronata, 

 l3ut difi'ers from it in several points of internal structure. At first I was inclined to 

 place it as a variety of that species, but have now decided to treat it as distinct. 



The musculature is peculiar, and difi'ers from that of the last species ; it is shown 

 in fig. 13 on Plate VIII. The first and second bands are short, and the second 

 extends much farther ventraUy than dorsally. The remaining bands (PI. VIII. fig. 13, 

 .3-7) are very long, extending almost to the median ventral line, and not fusing with 

 their neighbours dorsally. The curved muscle bands around the ends of the branchial 

 and atrial apertures (PL VIII. fig. 13, hr. m., at. m.) are stout and of a yellowish 

 colour, and are forked at their ends. Finally, the slender bands in front of the 

 atrial aperture are composed each of a single muscle fibre, and are interrupted in the 

 median dorsal line. 



The minute structure of the muscles in this specimen is easily made out. The 

 fibres are very distinct, and are cross striated (PI. VIII. fig. 11). They have each a 

 number of large distinct nuclei placed in single file. AVhen more highly magnified 

 (PI. VIII. fig. 12), the cross striation is seen to be due to the presence of a series of 

 quadi-angular bodies placed in a single row along each of the fibrils of which the muscle 

 fibres are composed. The nucleus of a fibre covers the breadth of four or five fibrils. 

 By examining very young specimens of Salpa democratica-mucronata it is possible to 

 trace the formation of a muscle band out of a few rows of ordinary fusiform mesoblast 



