A TAXONOMIC STUDY ON" PYROSOMA — METCALF AND HOPKINS. 203 



All lie, in general, transverse to the long axis of the body. There are 

 oral and atrial sphincter muscles in all species. A little behind the 

 mouth and encircling the prebranchial chamber, are one to several 

 fine bands or fibers, which we may call the circum-oral muscles (c. m.). 

 In the Pyrosomata fixata there is a system of branching fibers not found 

 in the other sub-genus, the so-called lateral muscular system. It con- 

 sists of two transverse strands, a posterior one (Z. m.') crossing in 

 front of the ganglion, and an anterior one (I. m.) in front of the en- 

 dostyle, both sets branching somewhat on each side of the body. 

 A pair of transverse, cloacal muscles (cl. m .) are found in all Pyrosomas. 

 In the Pyrosomata ambvlata these lie one on each side of the common 

 cloaca (fig. 37, pi. 30) ; in the Pyrosomata fixata they lie over the 

 middle of the peribranchial sacs (fig. 3, pi. 16). The cloacal muscles 

 of one zooid are connected with those of adjacent zooids by means of 

 the test-fibers already referred to. 



The oral aperture leads directly into the pharynx. It is circular 

 and is reinforced and held in position by a projecting shelf of the test. 

 The inner epithelium inside the mouth is produced into a number of 

 tentacle-like processes arranged in a ring. In the Pyrosomata ambu- 

 lata only the median, ventral process can be regarded as a true tentacle 

 (fig. 13, pi. 22); the others are really nothing more than thickened 

 folds in the edge of the mouth, although in some species they are 

 quite prominent. In the members of the other subgenus all of these 

 processes are true tentacles (fig. 2, pi. 16). They are each supplied 

 with nerve fibers and are probably sensory, for sensory cells have 

 been discovered in the similarly placed tentacles of other Tunicates 

 (as Molgula, Hunter, 1898) - 1 The median, large, ventral tentacle is 

 said by Ussow (187-6) to contain otoliths in the expanded, vesicle- 

 like portion at its base, but, on the basis of our own and other's 

 observations, this report seems to be mistaken. Ussow's descrip- 

 tion, if it were accurate, would indicate that the tentacle functions 

 as an organ of direction, through perception of gravity. Joliet (1888) 

 and Salensky (1891) suggest that by distending its basal vesicle with 

 blood the tentacle may be erected so as to close the mouth. 



The pharynx shows two portions, the prebranchial chamber 

 (buccal cavity) anterior to the peripharyngeal bands and behind 

 this the branchial sac whose lateral walls form the so-called branchial 

 basket, the respiratory area of the pharynx. The prebranchial 

 chamber is sometimes very short, but in the majority of species it is 

 more or less elongated to form an oral siphon. The water which 

 passes through the mouth into the pharynx makes its way out 

 through the stigmata into the right and left peribranchial sacs, 



» Compare the less conclusive studies upon Doliolum by Keferstein and Ehlcrs (1861), Grobbcn (1832), 

 and Uljanin (1884), and Neumann's discussion of the conditions in Pyrosoma (1909-13, p. 57). 



