334 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
such structures are discernible in the present species. The c7//ated 
sac (‘tubercule antérieur’ of Savigny)—an organ of universal occur- 
rence among Ascidians—is in Pyrosoma giganteuim an elongated, 
laterally compressed, funnel-shaped bag, about ;3,th of an inch long, 
which lies in the sinus, and, passing obliquely forwards and towards 
the hemal side, opens as above described. Its aperture has 
somewhat prominent lips, and is rather narrower than its upper 
portion, The posterior end of the sac appears to terminate cecally, 
and is applied against the posterior surface of the ganglion. The 
middle of the hemal side of the sac sometimes appears to be 
connected with a spheroidal tubercle, whose axis forms nearly a right 
angle with that of the sac. 
The muscular system is exceedingly simple in this species of Pyro- 
soma, consisting, besides the oral sphincter and buccal muscles 
already menticned, of only an atrial sphincter and the ‘ mid-atrial” 
muscles. 
The atrial aperture (fig. 7) is even smaller than the oral, not 
measuring more than from ,;th to sth of an inch in diameter. 
Radiating striz diverge from its margin on the surface ot the test, 
which, as at the oral aperture, forms a thick lip, and is continued for 
some little distance inwards upon the wall of the mid-atrium. A 
sphincter formed of pale smooth fibres, and constituting a circular 
band zt¢s5th of an inch in diameter, is developed at the junction of 
the external and atrial tunics. There is a similar but less dis- 
tinct appearance of radiating fibres to that exhibited at the oral 
sphincter. 
The mid-atrial muscles (gz) are broad flat bands of smooth 
muscular fibres, which lie in close contact with, and apparently 
attached to, the atrial tunic. One of these bands occupies about the 
middle two-fourths of the height of each lateral wall of the mid- 
atrium, and has a direction perpendicular to the axis of the ascidio- 
zooid. The bundle of fibres spreads out a little at each end, and then 
seems to be inserted by a sort of tendon into the outer tunic. Close 
to this tendinous insertion, at either end, a bundle of fibres (whether 
merely fibrous or muscular I cannot say) arises, and passes, partly to 
the nearest similar insertion of one of the mid-atrial muscles of the 
ascidiozooid above or below, partly to the same point of the mid-atrial 
muscle of some other ascidiozooid. Hence, when the wall of the 
ascidiarium is viewed from within, it presents such an interlacement 
of fibres as that exhibited in fig. 9. 
These muscles, in contracting, must tend to diminish the capacity 
of the atrium of the ascidiozooid to which they belong, and, if they all 
