AND DEVELOPMENT OF PYROSOMA. 
!> 
funnel-shaped bag, about yj^th of an inch long, which lies in the sinus, and, passing 
obliquely forwards and towards the haemal side, opens as above described. Its aperture 
has somewhat prominent lips, and is rather narrower than its upper portion. The poste- 
rior end of the sac appears to terminate ciecally, and is applied against the posterior sur- 
face of the ganglion. The middle of the haemal side of the sac sometimes appears to be 
connected with a spheroidal tubercle, whose axis forms nearly a right angle with thai of 
the sac. 
The muscular system is exceedingly simple in this species of JPyrosonm, consisting, 
besides the oral sphincter and buccal muscles already mentioned, 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 
sJoth to 5-Joth of an inch in diameter. Radiating striae diverge from its margin on t he 
surface of 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 r^s^h of an inch in diameter, is developed 
at the junction of the external and atrial tunics. There is a similar but less distinct 
appearance of radiating fibres to that exhibited at the oral sphincter. 
The mid-atrial muscles (g 2) 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 m id-atrium, 
and has a direction perpendicular to the axis of the ascidiozooid. The bundle of fibres 
spreads out a little at each end, and then seems to be inserted b} 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 act together, to shorten and narrow the 
ascidiarium. I do not suppose that their effect ill the latter direction can be very great ; 
but it might well be sufficient to account for the slight contraction of the whole ascidi- 
arium, and consequent retrogressive motion, observed by Peron and others. 
In my previous memoir, I have pointed out that the round, granular, yellowish patches 
on each side of the entrance of the branchial sac, and opposite the middle of the peri- 
pharyngeal ridge, are not, as Savigny imagined, the ovaries. I am greatly inclined to 
regard them as renal organs, but for the present defer the discussion of their structure 
and functions. 
The reproductive organs of each ascidiozooid of Pyrosoma may be divided into actual 
and potential— the genitalia of the ascidiozooid itself, and the blastema whence the 
genitalia of its buds will take their origin. I shall call this last the generative blastema ; 
while the genitalia proper are divisible, as I have already pointed out, into a single ovisac 
and a single testis. Both ovisac and testis are situated in the left five-sixths of the roof 
VOL. XXIII. 
2 1 
